****************************
                  *                          *
                  *       Update notes       *
                  *                          *
                  *    PYTHIA version 6.4    *
                  *                          *
                  ****************************
 
                   (Last updated 05 Sep 2013)
 
PYTHIA version 6.4 is a direct continuation of version 6.3;
actually 6.400 is identical with 6.327. Therefore it should not 
be a big operation for the normal user to run the program.
  
PYTHIA 6.4 comes with an updated big manual, reflecting the status
as of version 6.400. These update notes document what is new in 
each subsequent subversion.
  
-----------------------------------------------------------------------

TUNES OF FRAGMENTATION, UNDERLYING-EVENT, AND MINIMUM-BIAS PARAMETERS

  - From PYTHIA version 6.408, several sets of widely used PYTHIA parameter 
    settings ('tunes') have been collected and made more easily available 
    through the auxiliary routine PYTUNE(ITUNE), which should be called 
    before the call to PYINIT. Alternatively, since version 6.413, the 
    desired tune number can just be given in MSTP(5), in which case PYINIT
    will call PYTUNE automatically. 
  - In general, newer tunes are more reliable than older ones. To help 
    comparing the dates of different tunes, a rough date of first appearance
    or publication is given for each tune below. 
  - The following values of MSTP(5) / PYTUNE(ITUNE) are recognized (note that
    some of the newest tunes are only available in later PYTHIA versions. See
    the detailed update notes below or the printout from PYTUNE to see if a 
    specific tune is implemented in your PYTHIA version):
   
    ITUNE    NAME 
    ---------------------------------------------------------------------
        0 Default : No settings changed => PYTHIA defaults.
    =====================================================================

  - 100-199 : Old UE, Q2-ordered showers 
    ---------------------------------------------------------------------
     1st generation: Rick Field's CDF tunes and a few more
    ---------------------------------------------------------------------
     100       A : Rick Field's CDF Tune A                     (Oct 2002)
     101      AW : Rick Field's CDF Tune AW                    (Apr 2006)
     102      BW : Rick Field's CDF Tune BW                    (Apr 2006)
     103      DW : Rick Field's CDF Tune DW                    (Apr 2006)
     104     DWT : As DW but with slower UE ECM-scaling        (Apr 2006)
     105      QW : Rick Field's CDF Tune QW using CTEQ6.1M        (date?)
     106 ATLAS-DC2: Arthur Moraes' (old) ATLAS tune ("Rome")      (date?)
     107     ACR : Tune A modified with new CR model           (Mar 2007)
     108      D6 : Rick Field's CDF Tune D6 using CTEQ6L1         (date?)
     109     D6T : Rick Field's CDF Tune D6T using CTEQ6L1        (date?)
    ---------------------------------------------------------------------
     2nd generation: The same with Professor's LEP tune
    ---------------------------------------------------------------------
     110   A-Pro : Tune A, with LEP tune from Professor        (Oct 2008)
     111  AW-Pro : Tune AW, -"-                                (Oct 2008)
     112  BW-Pro : Tune BW, -"-                                (Oct 2008)
     113  DW-Pro : Tune DW, -"-                                (Oct 2008)
     114 DWT-Pro : Tune DWT, -"-                               (Oct 2008)
     115  QW-Pro : Tune QW, -"-                                (Oct 2008)
     116 ATLAS-DC2-Pro: ATLAS-DC2 / Rome, -"-                  (Oct 2008)
     117 ACR-Pro : Tune ACR, -"-                               (Oct 2008)
     118  D6-Pro : Tune D6, -"-                                (Oct 2008)
     119 D6T-Pro : Tune D6T, -"-                               (Oct 2008)
    ---------------------------------------------------------------------
     3rd generation: Complete Q2-ordered Tune by Professor
    ---------------------------------------------------------------------
     129 Pro-Q20 : Professor Q2-ordered tune                   (Feb 2009)
    =====================================================================

  - 200-299 : Intermediate and Hybrid Models
    ---------------------------------------------------------------------
     200    IM 1 : Intermediate model: new UE, Q2-ord. showers, new CR
     201     APT : Tune A w. pT-ordered FSR                    (Mar 2007)
     211 APT-Pro : Tune APT, with LEP tune from Professor      (Oct 2008)
     221 Perugia APT  : "Perugia" update of APT-Pro            (Feb 2009)
     226 Perugia APT6 : "Perugia" update of APT-Pro w. CTEQ6L1 (Feb 2009)
    =====================================================================

  - 300-399 : New UE, interleaved pT-ordered showers, annealing CR
    ---------------------------------------------------------------------
     1st generation: Sandhoff-Skands CDF Min-Bias tunes and a few more
    ---------------------------------------------------------------------
     300      S0 : Sandhoff-Skands Tune using the S0 CR model  (Apr 2006)
     301      S1 : Sandhoff-Skands Tune using the S1 CR model  (Apr 2006)
     302      S2 : Sandhoff-Skands Tune using the S2 CR model  (Apr 2006)
     303     S0A : S0 with "Tune A" UE energy scaling          (Apr 2006)
     304    NOCR : New UE "best try" without col. rec.         (Apr 2006)
     305     Old : New UE, original (primitive) col. rec.      (Aug 2004)
     306 ATLAS-CSC: Arthur Moraes' (new) ATLAS tune w. CTEQ6L1    (date?)
    ---------------------------------------------------------------------
     2nd generation: The same with Professor's LEP tune
    ---------------------------------------------------------------------
     310   S0-Pro : S0 with updated LEP pars from Professor    (Oct 2008)
     311   S1-Pro : S1 -"-                                     (Oct 2008)
     312   S2-Pro : S2 -"-                                     (Oct 2008)
     313  S0A-Pro : S0A -"-                                    (Oct 2008)
     314 NOCR-Pro : NOCR -"-                                   (Oct 2008)
     315  Old-Pro : Old -"-                                    (Oct 2008)
     316 ATLAS MC08: 2008 ATLAS tune w. CTEQ6L1                    (2008)
    ---------------------------------------------------------------------
     3rd generation: The Perugia, MC09, and Professor pT-ordered Tunes
    ---------------------------------------------------------------------
     320 Perugia 0 : "Perugia" update of S0-Pro                    (2009)
     321 Perugia HARD : More ISR, More FSR, Less MPI, Less BR, Less HAD
     322 Perugia SOFT : Less ISR, Less FSR, More MPI, More BR, More HAD
     323 Perugia 3 : Alternative to Perugia 0, with different ISR/MPI
                     balance & different scaling to LHC & RHIC     (2009)
     324 Perugia NOCR : "Perugia" update of NOCR-Pro               (2009)
     325 Perugia * : "Perugia" Tune w. (external) MRSTLO* PDFs     (2009)
     326 Perugia 6 : "Perugia" Tune w. (external) CTEQ6L1 PDFs     (2009)
     327 Perugia 2010: Alternative to Perugia 0, with more FSR off ISR,
                     more Nch at lower Ecm and more strangeness    (2010)
     328 Perugia K : Alternative to Perugia 2010, with a K-        (2010)
                     factor applied to MPI cross sections
     329 Pro-pT0   : Professor pT-ordered tune w. S0 CR model      (2009)
     330 ATLAS MC09  : 2009 ATLAS tune w. LO*                      (2009)
     331 ATLAS MC09c : 2009 ATLAS tune w. LO*, retuned CR          (2009)
     334 Perugia NOCR : Perugia 2010 variant of NOCR               (2010)
     335 Pro-PT*   : Professor pT-ordered tune w. LO* PDFs         (2009)
     336 Pro-PT6   : Professor pT-ordered tune w. CTEQ6L1 PDFs     (2009)
     339 Pro-PT**  : Professor pT-ordered tune w. LO** PDFs        (2009)    
    ---------------------------------------------------------------------
     4th generation: tunes incorporating 7- and 8-TeV data
    ---------------------------------------------------------------------
     340 AMBT1       : 1st ATLAS tune incl 7 TeV, w. LO* PDFs      (2010)    
     341 Z1          : Retune of AMBT1 by Field w CTEQ5L PDFs      (2010)
     342 Z1-LEP      : Retune of Z1 by Skands w CTEQ5L PDFs        (2010)
     343 Z2          : Retune of Z1 by Field w CTEQ6L1 PDFs        (2010)
     344 Z2-LEP      : Retune of Z1 by Skands w CTEQ6L1 PDFs       (2010)
     345 AMBT2B-CT6L : 2nd ATLAS MB tune, vers 'B', w CTEQ6L1      (2011)
     346 AUET2B-CT6L : UE tune accompanying AMBT2B                 (2011)
     347 AUET2B-CT66 : AUET2 with CTEQ 6.6 NLO PDFs                (2011)
     348 AUET2B-CT10 : AUET2 with CTEQ 10 NLO PDFs                 (2011)
     349 AUET2B-NN21 : AUET2 with NNPDF 2.1 NLO PDFs               (2011)
     350 Perugia 2011 (P11) : Retune of Perugia 2010 incl 7-TeV data (2011)
     351 P11-radHi   : Variation with alphaS(pT/2) 
     352 P11-radLo   : Variation with alphaS(2pT)
     353 P11-mpiHi   : Variation with more semi-hard MPI
     354 P11-noCR    : Variation without color reconnections
     355 P11-LO**    : Perugia 2011 using MSTW LO** PDFs           (2011)
     356 P11-C6      : Perugia 2011 using CTEQ6L1 PDFs             (2011)
     357 P11-T16     : Variation with PARP(90)=0.16 away from 7 TeV
     358 P11-T32     : Variation with PARP(90)=0.32 awat from 7 TeV
     359 P11-TeV     : Perugia 2011 optimized for Tevatron         (2011)
     360 S Global    : Schulz-Skands Global fit                    (2011)
     361 S 7000      : Schulz-Skands at 7000 GeV                   (2011)
     362 S 1960      : Schulz-Skands at 1960 GeV                   (2011)
     363 S 1800      : Schulz-Skands at 1800 GeV                   (2011)
     364 S 900       : Schulz-Skands at 900 GeV                    (2011)
     365 S 630       : Schulz-Skands at 630 GeV                    (2011)
     370 Perugia 2012 (P12) : Retune of Perugia 2011 w CTEQ6L1     (2012)
     371 P12-radHi   : Variation with alphaS(pT/2) 
     372 P12-radLo   : Variation with alphaS(2pT)
     373 P12-mpiHi   : Variation with more semi-hard MPI -> more UE
     374 P12-loCR    : Variation using lower CR strength -> more Nch
     375 P12-noCR    : Variation without any color reconnections
     376 P12-FL      : Variation with more longitudinal fragmentation
     377 P12-FT      : Variation with more transverse fragmentation
     378 P12-M8LO    : Variation using MSTW 2008 LO PDFs     
     379 P12-LO**    : Variation using MRST LO** PDFs     
     380 P12-MB2     : same as Perugia 2012, with PARP(87)=0D0     (2013)
     381 P12-ueHi    : Variation with higher UE (lower pT0)        (2013)
     382 P12-ueLo    : Variation with lower UE (higher pT0)        (2013)
     383 P12-IBK     : Perugia 2012 with IBK ee fragmentation parameters
     390 IBK-CTEQ5L  : Innsbruck ee+pp tune with CTEQ5 LO PDFs     (2013)
     391 IBK-CTEQ6LL :       with CTEQ6LL LO PDFs
     392 IBK-MSTW08LO:       with MSTW08 LO PDFS
     393 IBK-CTEQ66NLO:      with CTEQ66 NLO PDFs
     394 IBK-CT10NLO :       with CT10 NLO PDFs
     395 IBK-MSTW08NLO:      with MSTW08 NLO PDFs
     396 IBK-MSTW08LO*:      with MSTW07 LO* PDFs
     397 IBK-MRSTLO**:       with MRSTMCal (LO**) PDFs
     398 IBK-CT09MC2 :       with CTEQ09MC2 PDFs  
    =====================================================================

  - 1200-1499 : The Uppsala SCI and GAL models
    ---------------------------------------------------------------------
    1201 SCI 0 : Soft-Colour-Interaction model Org pars.           (1998)
    1202 SCI 1 : SCI 0. Tevatron MB retuned (Skands)               (2006)
    1401 GAL 0 : Generalized area-law model. Org pars              (1998)
    1402 GAL 1 : GAL 0. Tevatron MB retuned (Skands)               (2006)
    =====================================================================
    
-----------------------------------------------------------------------

NOTE ON CHANGING THE SIZE OF THE EVENT RECORDS IN PYTHIA

  Concerns the common blocks: /HEPEVT/, /PYJETS/, /PYCTAG/

  For very high energies, or to include pile-up, it may be expedient
  to increase the size of PYTHIA's event records. In order to avoid
  internal inconsistencies, it is important that this be done in the
  right way. In the PYTHIA 6.4 manual, the description of the
  necessary steps only fully applies to the so-called old MPI
  framework. Here, we give the full set of steps that allow to change
  the size of the event record consistently, also for the new
  framework. 

  There are essentially 2 ways of doing this. 

    1. The most stable (and hence recommended) method is to avoid
    tampering with the internal PYTHIA common blocks at all, and
    instead only change the size of the /HEPEVT/ common block that is
    used to interface external programs.  In this mode, PYTHIA will
    generate each event using its normal size-4000 event record, but
    several such events can be added, one after the other, in the
    larger /HEPEVT/ common block, e.g. to simulate pile-up. This is
    the solution adopted by GENSER for the modified PYTHIA versions it
    provides to CERN users. 

    2. A more far-reaching change is to change the internal size of
    all the size-4000 arrays in the /PYJETS/ common block everywhere
    in the code. In this case, also MSTU(5) has to be updated 
    to the new value, as described in the manual. If one wishes to
    interface external programs using /HEPEVT/, obviously the size of
    the arrays in this common block will also have to be changed. This
    is sufficient to use the old MPI framework with the modified
    event-record size. In order to use the new MPI framework as well,
    the size-4000 arrays in the /PYCTAG/ common block also have to be
    changed, and the local size-4000 arrays defined in the routine
    PYFSCR likewise have to be changed to the new event-record size,
    in order to eliminate any possibility for out-of-bounds crashes. 

-----------------------------------------------------------------------

THE UNIVERSAL EXTRA DIMENSIONS SCENARIO

Starting with version 6.4.17 Pythia includes an implementation of the
UED model. This implementation is courtesy of  M.Elkacimi, D.Goujdami,
H.Przysiezniak, see [hep-ph/0602198] (Les Houches 2005). The following
gives a rough physics and user's manual.

Theories with extra dimensions offer a description of the gravitational
interaction at low energy, and thus have received considerable attention.
One very interesting incarnation was formulated by T. Appelquist, H.-C.
Cheng and B.A. Dobrescu in Phys. Rev. D64 (2001) 035002, the Universal
Extra Dimensions (UED) model, where "Universal" comes from the fact that
all Standard Model (SM) fields propagate into the extra dimensions.

From version 6.4.17, the UED model has been implemented in PYTHIA
in its "minimal"  formulation, with one small extra dimension and with
the additional possibility of gravity mediated decays as described by
C. Macesanu, C.D. McMullen and S. Nandi in Phys. Lett B546 (2002) 253.

In the UED model, the SM lives in (4 + delta) space-time dimensions. This
effective theory is valid below some scale Lambda (cutoff scale). The 
compactification scale is 1/R < Lambda for the delta extra dimensions.
In Minimal UED, delta = 1. To avoid fine-tuning of the  parameters in the 
Higgs sector, 1/R should not be much higher than the electroweak scale.
The default values in Pythia are 1/R = 1 TeV and Lambda = 5 TeV.

In Minimal UED, each SM particle has n=1,2,3,... KK excitations, of 
squared mass m_n^2 = n^2/R^2 + m_SM^2 . Pythia so far only incorporates the
lowest-lying of these excitations. 

Momentum is conserved in the delta=1 extra dimension. In 3D, this implies 
conservation of the KK number, hence there is never a vertex with only one 
KK excitation, and KK particles are always produced in pairs.

To each fermionic chiral state corresponds a KK tower. Hence there are two
towers per quark flavour and per massive lepton and one tower per massless 
neutrino. Each EW boson has an associated KK tower, and the bosons mix 
within each level as in the SM. Also, each Higgs boson KK level is 
composed of one charged, one scalar CP-odd and one scalar CP-even Higgs bosons.
The interactions between the Higgs field, the gauge bosons and the fermions 
are described by the same couplings as the SM ones.

These new excitations have been defined following the PDG UED particle 
coding notation, although with a slight twist to it. Indeed, the PDG 
denotes the KK particles in the following manner (for the first level 
excitations only):

\begin{tabular}{lr@{\protect\rule[1mm]{0mm}{4mm}}}
$d_L^{(1)}$              & 5100001 \\
$u_L^{(1)}$              & 5100002 \\
$e_L^{(1)-}$             & 5100011 \\
$\nu_{eL}^{(1)}$         & 5100012 \\
$g^{(1)}$                & 5100021 \\
$\gamma^{(1)}$           & 5100022 \\
$Z^{(1)0}$               & 5100023 \\
$W^{(1)+}$               & 5100024 \\
$d_R^{(1)}$              & 6100001 \\
$u_R^{(1)}$              & 6100002 \\
$e_R^{(1)-}$             & 6100011 \\
\end{tabular}

where for the fermions, "L" and "R" are meant to reflect weak interaction 
quantum numbers of the lowest level SM particles associated to the KK
excitations, and denote whether the fermion is respectively a doublet or
a singlet under SU(2)_W. 

In Pythia, to avoid any confusion with helicity states, we use the 
the notation "D"oublet and "S"inglet instead of "L" and "R", and we
use * rather than ^(1) to denote that the state is an excitation, 
as follows (for the first level excitations only):

\begin{tabular}{lr@{\protect\rule[1mm]{0mm}{4mm}}}
$d*_D$			 & 5100001 \\
$u*_D$			 & 5100002 \\
$e*_D-$			 & 5100011 \\
$nue*_D$		 & 5100012 \\
$g*$			 & 5100021 \\
$gamma*$		 & 5100022 \\
$Z*0$			 & 5100023 \\
$W*+$			 & 5100024 \\
$d*_S$			 & 6100001 \\
$u*_S$			 & 6100002 \\
$e*_S$			 & 6100011 \\
\end{tabular}

In either case (PDG or Pythia notation), both the doublet and singlet
UED excitations are ordinary Dirac fermions, which have both left and
right handed chiral spinor components. 

If there is no mixing between the doublets and singlets, then these are 
also the mass eigenstates. One can safely neglect mixing for all but the
top quark. In Pythia 6.4.17, mixing has been neglected for all
quarks. Top mixing matrices are foreseen to be added in a later version.

* RADIATIVE CORRECTIONS TO THE UED PARTICLE EXCITATION MASSES

The mass spectrum at each KK level is highly degenerate except for 
particles with large zero mode masses (t, W, Z, h). First order
radiative corrections are included in the implementation and are
usually of order 20% for strongly interacting particles (the heaviest
being the gluon), and less than 10% for leptons and EW bosons (the
lightest being the photon). With these mass splittings, the SM quark
and gluon KK excitations will cascade decay down to the "Lightest KK
Particle" (LKP), unless there is another mechanism through which the
particles can decay. See the article by H.-C.Cheng, K.T.Matchev and
M.Schmaltz in Phys.Rev. D66 (2002) 036005 for the radiative
corrections to the masses. 

* GRAVITY MEDIATED DECAYS

If the 4+1-dimensional "UED" space is embedded into a larger space of
(4+1+N) dimensions, with N a number of additional extra dimensions 
of size eV^{-1} into which only gravity propagates, then gravity
mediated decays also become possible.  
 
The graviton field appears as a massless graviton with an infinity of 
excited modes whose masses differ by O(eV). The graviton couples to all 
KK particles which can decay through KK number violating interactions
mediated by gravity, by emitting a graviton e.g. gamma* -> gamma + G .
See the article by A.DeRujula, A.Donini, M.B.Gavela and S.Rigolin in
Phys.Lett. B482(2000) 195 for the gravity mediated decay widths 
expressions. These widths depend on 1/R as well as on N.

In this scenario, an interplay occurs between the mass splitting decay 
widths and the gravity mediated decay widths. If 
       Gamma(mass splitting) > Gamma(gravity mediated)
then the gluon and quark excitations will cascade down to the excited
photon gamma*, which in turn will decay into a photon and graviton.
See the article by C.Macesanu, C.D.McMullen and S.Nandi in Phys.Lett. B546
(2002) 253 for the decay widths plots versus 1/R. One can see from these
plots that for N=6 large extra dimensions (the default in Pythia),
this type of cascade occurs for 1/R <= 2TeV. 

In Pythia 6.4.17, we enable the gravity mediated decays only for the
level 1 excited photon. Gravity mediated decays  for all particles
is foreseen to be included in a future version, and the routine to
compute the relevant widths is already present in the code, but
presently can only be called standalone. 

* FEATURES OF THE UED IMPLEMENTATION

The UED free parameters are 1/R, N, Lambda, the number of quark flavours 
and m_H. In the code, the switches and parameters are stored in the
new common block:

      COMMON/PYPUED/IUED(0:99),RUED(0:99)

with the following switches and parameters so far defined

     IUED(1) UED ON(=1)/OFF(=0) switch 
             Default value = 0
     IUED(2) ON(=1)/OFF(=0) switch for gravity mediated decays
             Default value = 0
     IUED(3) NFLAVOURS Number of KK excitation quark flavours
             Default value = 5
     IUED(4) N the number of non-universal large extra dimensions
             Default value = 6
     IUED(5) Selects whether the code takes Lambda (=0)
             or Lambda*R (=1) as input.
             Default value = 0
     IUED(6) Radiative corrections to the masses ON (=1) or OFF (=0)
             Default value = 1

     RUED(1) RINV (1/R) the curvature of the extra dimension
             Default value = 1000 GeV
     RUED(2) XMD the (4+N)-dimensional Planck scale
             Default value = 5000 GeV
     RUED(3) LAMUED (Lambda cutoff scale), used for IUED(5)=0
             Default value = 20000 GeV
     RUED(4) LAMUED/RINV (order of 10-20), used for IUED(5)=1
             Default value = 20     

       N.B. The Higgs mass is also a free parameter
       of the UED theory but is set through PMAS(25,1).

Note: level-1 KK excitations of all Standard Model fermions and gauge
bosons are present, but the additional Higgs bosons are not present in
the current implementation.

Nine new production processes have been added:

ISUB  production process (ref: hep-ph/0201300 and Azuelos-Beauchemin)
---- ---------------------------------
 311 g   + g      -> g*    + g*
 312 g   + q      -> g*    + q*_D/q*_S
 313 q_i + q_j    -> q*_Di + q*_Dj
                  -> q*_Si + q*_Sj
 314 g   + g      -> q*_D  + q*_Dbar
   	          -> q*_S  + q*_Sbar
 315 q   + qbar   -> q*_D  + q*_Dbar
	 	  -> q*_S  + q*_Sbar
 316 q_i + q_barj -> q*_Di + q*_Sbarj
 317 q_i + q_barj -> q*_Di + q*_Dbarj
		  -> q*_Si + q*_Sbarj
 318 q_i + q_j    -> q*_Di + q*_Sj
 319 q_i + q_bari -> q*_Dj + q*_Dbarj


The following mass-splitting decay modes for n=1 are also included 
(ref: hep-ph/0205314):
     l*_S  -> l   + gamma*
     q*_S  -> q   + gamma*
     q*_S  -> q   + Z*
     l*_D  -> l   + gamma* 
     nu*_D -> nu  + gamma*
     q*_D  -> q   + gamma*
     q*_D  -> q   + Z*
     q*_Di -> q_j + W*
     W*+-  -> l+- + nu*_D
	   -> nu  + l*_D+-
     Z*    -> nu_bar + nu*_D
	   -> l-+ + l*_D+-
     g*    -> q   + q*_Dbar
	   -> q   + q*_Sbar

The only gravity mediated decay which is turned is is the excited photon:

     gamma*-> gamma + Graviton

Four new subroutines have been added to handle UED-specific tasks:

      SUBROUTINE PYXDIN   to initialize Universal Extra Dimensions  
      SUBROUTINE PYUEDC   to compute UED mass radiative corrections
      SUBROUTINE PYXUED   to compute UED cross sections      
      SUBROUTINE PYGRAM   to generate UED graviton mass spectrum 

In addition, several Pythia routines have been modified to include the
UED implementation. These modified routines include

      SUBROUTINE PYGIVE   now accepts input also for IUED and RUED  
      SUBROUTINE PYINIT   added call to PYXDIN to initialize UED 
      SUBROUTINE PYMAXI   small extension for UED overestimates
      SUBROUTINE PYPTFS   small extension for showering KK gluons
      SUBROUTINE PYRAND   extended to choose flavours in UED processes
      SUBROUTINE PYRESD   added call to PYGRAM to choose graviton mass
                          from continuous spectrum in UED decays to
                          gravitons 
      SUBROUTINE PYSCAT   extended to include UED processes
      SUBROUTINE PYSIGH   small extension to call PYXUED for UED
      SUBROUTINE PYWIDT   extended to compute KK decay widths

-----------------------------------------------------------------------
 
CHANGES IN CONSECUTIVE SUBVERSIONS

6.4.00 : 25 March 2006
  - This version is identical with Pythia 6.327. Readers should therefore
    turn to the Pythia 6.3 update notes to trace the evolution of the
    program.
  - The only changes are updates of character strings, 6.3 -> 6.4.

6.4.01 : 16 April 2006
  - Bug fix in PYRAND: when the Les Houches Accord interface is used 
    with more than 50 processes, some of them would be mapped into 
    a range of PYTHIA process numbers assigned to SUSY processes, 
    where KFPR(ISUB,1) and KFPR(ISUB,2) would be overwritten. 
  - Modification of string fragmentation, introducing new tolerance
    parameter.
    PARU(14): (D=2.) when passing string corners, the (mis)match of
        transverse momentum directions may need to be compensated
        by using momentum fractions x outside the allowed range 
        0 < x < 1, by having a slightly negative x. Occasionally 
        the x can become quite negative, and then rarely give strange 
        results. The new parameter sets limits how far outside the
        allowed range one may go before rejecting the current try
        and restarting the fragmentation of the current string.
  - The size of the /PYPART/ commonblock has been expanded, by 
    replacing the current MAXNUP size parameter by a new MAXNUR one
    with size 1000. This should avoid some problems with overflows.
  - Bug correction: for the MSTP(95) > 3 options the colour 
    reconnection in PYFSCR was not called from PYEVNW.

6.4.02 : 2 May 2006
  - PYFSCR: bugfix and updates (performing colour reconnections in the
    new underlying-event framework). A bug for MSTP(95)=2,3 caused an 
    unintended suppression of connections between free colour octets. 
    This has now been fixed, so the scenario works as described in the 
    manual. In addition, a new option has been added, by MSTP(95)=6,7, 
    where only partons with one and only one free colour tag are allowed 
    to initiate a string piece. This amounts to saying that the string 
    collapse is driven by free triplet charges. Finally, the parameter 
    PARP(78) now also works for MSTP(95)>1, and controls the strength of 
    the colour reconnections. In addition, the reconnection probability 
    grows as a function of the number of multiple interactions, to reflect
    the possibility of a higher amount of reconnections occurring in  
    collisions with a higher amount of strings. The full description of 
    the PYFSCR parameters is thus now:
    MSTP(95) : CR Type
             = 1  : old cut-and-paste reconnections, handled in PYMIHK
             = 2  : Type I(no gg loops); hadron-hadron only
             = 3  : Type I(no gg loops); all beams
             = 4  : Type II(gg loops)  ; hadron-hadron only
             = 5  : Type II(gg loops)  ; all beams
             = 6  : Type S             ; hadron-hadron only 
             = 7  : Type S             ; all beams
    Types I and II are described in Sandhoff+Skands, in hep-ph/0604120.
    Type S is driven by starting only from free triplets, not octets.
    PARP(78) : CR Strength 
           The probability to keep a given colour-dipole pair depends 
           on PARP(78) and the number of interactions, MINT(31):
                  PKEEP = (1D0-PARP(78))**(MINT(31)) ;
           PARP(78) -> 1D0 : Full strength. 
           PARP(78) -> 0D0 : Zero strength. No reconnections.    
    NOTE: New min-bias/UE tunes with these new models are also available 
    from the web page.
  - Sped-up machinery for minimum-bias events, ISUB = 95.
    Specifically, the simultaneous choice of impact parameter b and
    pT for the hardest interaction has been optimized. The key trick 
    is to split the b range in two, roughly where the probability for
    an interaction of two hadrons passing through each other has 
    dropped to 1/2. With such a split it is feasible to pick b once
    and for all, and then to use two different formalisms for the pT 
    selection. At small b, one includes the "Sudakov" exponent to
    generate a trial sequence of falling pT's until accepted. If the
    sequence reaches pT = 0 the generation is restarted at maximum.
    At large b the trial pT's are selected unordered over the whole
    pT range, and then the "Sudakov" exponent is used as (part of) 
    the acceptance weight. This technical change should not affect 
    physics.   
    Internally, two new variables are used:
    MINT(39) : region of b value in current event.
        = 0 : not defined (e.g. for MSTP(82) = 1 or 2).
        = 1 : low-b region.
        = 2 : high-b region.
    VINT(139) : current b value. Note that no rescaling to physical
        units is performed, but only to an average b of 1 in 
        minimum-bias events, and thereby below 1 for high-pT events.
        (Related to the jet pedestal effect.) Thus it only has a 
        relative meaning in the context of comparing different events 
        inside the same sample (i.e. generated with the same parameter 
        values).  
  - Initialization of multiple interactions did not take into account
    that the old and new multiple interactions scenarios use somewhat
    different factorization scale choices. This gave partly misleading
    initialization information for the new scenarios, but hardly affects 
    the events actually generated for Tevatron or LHC applications. It
    could be more visible if tunes are extended to lower energies 
  - PYSLHA: update. When reading in SLHA decay tables for Higgs bosons, a
    message is printed to alert the user to the fact that the h/H/A->gg
    decay width is also used to compute the cross section for the inverse 
    process: gg -> h/H/A
  - PYFEYN: update. The previous interface to FeynHiggs became obsolete 
    with FeynHiggs version 2.3, where a number of parameters in the
    relevant FeynHiggs call routines were changed from DOUBLE PRECISION
    to DOUBLE COMPLEX. The corresponding change has now been introduced
    in PYFEYN as well, so that earlier Pythia versions are compatible 
    with FeynHiggs 2.2, while Pythia versions from 6.402 onward are 
    compatible with FeynHiggs 2.3.
  - PYSUGI: small bug fix, removed a CLOSE statement that could lead to
    crashes if no file had previously been opened.
  - Production channels g g  and gamma gamma -> Higgs rely on knowledge
    of the Higgs -> g g and gamma gamma partial widths. If SLHA is 
    used to replace the internal decay tables the program would be 
    looking in the wrong place. This has now been corrected. Note,
    however, that it is not possible to study e.g. g g -> Higgs if the
    new decay tables do not provide the Higgs -> g g decay channel.      

6.4.03 : 7 June 2006
  - The Pythia 6.4 Physics and Manual is now published in 
    T. Sjostrand, S. Mrenna and P. Skands, JHEP05 (2006) 026 
    [hep-ph/0603175]. 
    From now on, this is the only correct reference to the Pythia 6
    program.
  - PYFEYN: further bug correction in this interface.
  - Typo in the sped-up multiple-interactions treatment in 6.402 
    may affect results for the impact-parameter profile MSTP(82) = 5
    when PARP(83) < 2, and is now corrected.
  - Insert Planck scale in PYSUGI in appropriate units:
    AMPL = 2.4 E18 GeV = {8 pi G_newton}^{1/2}.
  - Remove superfluous comma in PYSUGI printout statement. 
  - Move default initialization of the PYLH3P and PYLH3C commonblocks
    to BLOCK DATA PYDATA for consistency with Fortran standard.
 -  PYSIGH: An additional possible classification of basic scattering
    processes now exists, for the special case of Weak Boson Fusion 
    and g g or q qbar -> Q Qbar Higgs processes:
    ISJETS = 2: Processes for which the matrix element does contains jets
      at leading order, but which are not included in ISJETS=1 since they
      are not of a QCD origin, as for instance the forward jets in
      WW/ZZ fusion to Higgs, qq -> qqH. The processes classified as 
      ISJETS=2 are currently ISUB=121-124, 173, 174, 178, 179, 181, 182, 
      186, 187. The maximum scale for initial state radiation in these 
      processes is set equal to the factorisation scale, see MSTP(32) 
      and MSTP(39).

6.4.04 : 10 August 2006
  - Changes to handle the new proposed Les Houches Event Files (LHEF) 
    format [hep-ph/0609017].
    1) The existing two sample routines UPINIT and UPEVNT, which read 
    the PYTHIA old separate initialization and event files, have been
    commented out (so the code is still there, just in case...).
    2) The new (uncommented)  UPINIT and UPEVNT routines handle an LHEF, 
    for initialization and event read-in. As before, the file to be used 
    must be opened by the user in the main program, and the file number 
    set in MSTP(161) and MSTP(162). Previously, with two separate input
    files, MSTP(161) and MSTP(162) were different. Now they are the same, 
    but both must still be set.
    (As before, the routines that come with  PYTHIA are only suggestions. 
    You are free to supply your own routines and link them instead. The 
    difference is that now there will be default code that eventually will 
    work right away with several different parton-level generators.)
    3) A new routine PYLHEF has been added to produce an LHE file from 
    a parton-level PYTHIA run output. Recall that, previously, PYUPEV
    had to be used instead of PYEVNT to produce parton-level-only events
    and write them to file MSTP(161), and a call to PYUPIN had to be added 
    at the end to write initialization information to file MSTP(162),
    both files to be opened by the user. (The PYUPIN call has to come 
    at the end, since it is only then that cross sections are known.)
    By adding a call to PYLHEF after the PYUPIN call, the two files
    will be combined into one single LHE file.
    MSTP(163) : (D=0) file unit number on which the LHE file will be 
        written. The corresponding file must be opened by the user in 
        the main program.
    MSTP(164) : (D=0) fate of input files when an LHE file is created by
        PYLHEF.
        = 0: the input files, associated with units MSTP(161) and
           MSTP(162), are removed.
        = 1: the input files are kept unchanged, although the information
           is now duplicated in the new file.
    4) Note: it would have been possible to avoid the use of two
    intermediate files, by a judicious use of dummy cross sections
    when the beginning of the file is first written, with the correct 
    ones substituted after a REWIND. This seemed like overkill, given the 
    small use of PYTHIA as a parton-level-only generator. The current
    facility to write LHE files is mainly intended to test the LHEF 
    structure. 
    5) The precision with which lifetime and spin are written in PYUPEV
    has been reduced, in particular since spin is not assigned by PYTHIA 
    and thus always defaults to 9. = unassigned,. and lifetime is 
    usually assumed 0. = vanishing.
  - The processes 381-388 for QCD processes modified by technicolor
    interactions are reclassified as being QCD processes in PYSIGH
    (except for top production), i.e. multiple interactions and 
    initial-state radiation are restricted to be softer than the 
    hard process itself.
  - The colour octet wavefunction-at-the-origin default values for 
    charmonia and bottomonia in PARP(141)-PARP(150) are now set 
    according to the values in 
      P. Nason et al., in "Standard model physics (and more) at the LHC",
      eds. G. Altarelli and M.L. Mangano, CERN 2000-004, p. 231
      [hep-ph/0003142]
    as described in talk by M. Bargiotti in the HERA4LHC workshop,
    CERN, 8 June 2006. 
  - The values of the parton densities, xf(x,Q2) of the two incoming 
    partons to the hard interaction (if any) are now stored in PARI(29) 
    and PARI(30), respectively. (And internally also in VINT(39) and 
    VINT(40). The XSFX array, which contains this kind of information,
    is overwritten for each new multiple interaction, and is therefore
    not so useful.) Since before the related flavours are stored in
    MSTI(15), MSTI(16), the x values in PARI(33) and PARI(34), and the
    Q scale in PARI(23). 
  - The size NMXHEP of the HEPEVT commonblock is now stored in MSTU(8)
    (for informational purposes).
  - The two dummy subroutines FHSETPARA and FHIGGSCORR have been updated
    to take complex variables, cf. update note 6.402 for PYFEYN.
    Irrelevant in practice, but introduced for consistency. 

6.4.05 : 31 August 2006
  - Mistake in 6.404 when processes 381-388 were to be classified as 
    QCD processes (an .OR. instead of an .AND.) lead to all processes
    being classified as QCD-type, and thereby, in some cases, to the 
    maximum scale for ISR and MI being set incorrectly.
  - Several processes involving photons in the initial, intermediate 
    or final state have been assigned ISJETS=3 in PYSIGH. This mean 
    that the maximum shower scale is set equal to the hard-process 
    scale, typically pT, rather than the full energy. Thereby (in most
    cases) the old behaviour from Pythia 6.2 should be recovered for 
    these processes. The ones relevant for hadron colliders are 10, 18, 
    19, 20, 83, 106 and 114. The ones for incoming photon beams are 
    33 - 36, 54, 58, 69, 70, 80, 84, 85, 107, 108, 110 and 131 - 140. 
  - A few processes have been added to the ISJETS = 2 class introduced 
    in 6.403, i.e. WW/ZZ fusion: 5, 8, 71, 72, 73, 76, 77, 351 and 352.
  - Showering and PDF scales for elastic and diffractive processes 
    explicitly set zero, for cosmetics reasons only.  
  - New option MSTP(123) = 3 gives same behaviour as = 2 but does not
    print out warnings.    

6.4.06 : 14 September 2006
  - In 6.402 a bug was fixed, where previously an incorrect PDF scale 
    choice was used at initialization of the new multiple interactions 
    scenario: intended to have pT^2 but got pT^2 + pT_0^2, as in the 
    old scenario. The mistake did not have so dramatic consequences, 
    since the intended scale was used during event generation, so it
    mainly affected the initialization of the impact-parameter picture.
    When the bug was fixed in 6.402, unfortunately the fix came to 
    overwrite the initialization of the old scenario, which therefore 
    came to suffer from the opposite problem, of being initialized with
    a PDF scale of pT^2 instead of the pT^2 + pT_0^2 intended for this 
    scenario. This is now fixed.
  - Second-order matching formulae for alpha_strong at flavour 
    thresholds introduced, following W.J. Marciano, Phys. Rev. D29 
    (1984) 580.

6.4.07 : 25 September 2006
  - New option for Les Houches Event Files.
    MSTP(165) : (D=0) writing or not of an extra line with parton 
        density information for each event, using the format suggested
        in the LHEF writeup:
        #pdf id1 id2 x1 x2 scalePDF xpdf1 xpdf2
        which here translates into
        #pdf MSTI(15) MSTI(16) PARI(33) PARI(34) PARI(23) PARI(29) PARI(30)
        = 0 : no.
        = 1 : yes, provided that MSTP(162) > 0.
        Note: currently such information is not read back in when the
            UPEVNT routine is used. It is read by Pythia8, however. 

6.4.08 : 24 October 2006
  - Colour Reconnections: 
    The new colour annealing models described in the 6.402 update notes 
    above were previously only available with the new interleaved 
    underlying-event framework (MSTP(81)=21). To help extricate genuine 
    colour-reconnection effects from changes in the shower and other 
    underlying-event aspects, the colour annealing models can now also be  
    used as an 'afterburner' to the old underlying-event framework (i.e. 
    with MSTP(81)=1). The models are switched on, as before, using MSTP(95),
    with values between 2 and 6 currently valid. The strength is regulated 
    with 0D0 < PARP(78) < 1D0. Since the models do increase the time it
    takes to generate an event, it may be of interest to note that MSTP(95)=6 
    is somewhat faster than the other models. In order not to 'double count' 
    colour reconnections, we also recommend setting the colour connection 
    parameter PARP(85)=0D0 when using the old framework with these models.
  - Pythia Tunes for Underlying Event (UE) and Min-Bias (MB):
    Several sets of widely used PYTHIA parameter settings ('tunes') have 
    been collected and made more easily available through the auxiliary 
    routine PYTUNE(ITUNE), which should be called before the call to 
    PYINIT. At this time, the following values of ITUNE are recognized:
    ITUNE    NAME 
        0 Default : No settings changed => linked Pythia version's defaults.
    ====== Old UE, Q2-ordered showers ========================================
      100       A : Rick Field's Tune A 
      101      AW : Rick Field's Tune AW
      102      BW : Rick Field's Tune BW
      103      DW : Rick Field's Tune DW
      104     DWT : Rick Field's Tune DW with slower UE energy scaling
      105      QW : Rick Field's Tune QW (NB: needs CTEQ6.1 pdfs externally)
      106   ATLAS : Arthur Moraes' ATLAS tune
      107     ACR : Tune A modified with annealing CR
    ====== New UE, Q2-ordered showers ========================================
      200    IM 1 : Intermediate 1: new UE, Q2-ordered showers, annealing CR
    ====== New UE, interleaved pT-ordered showers, annealing CR ==============
      300      S0 : Sandhoff-Skands Tune 0 
      301      S1 : Sandhoff-Skands Tune 1
      302      S2 : Sandhoff-Skands Tune 2
      303     S0A : S0 with "Tune A" UE energy scaling
      304    NOCR : New UE "best try" without colour reconnections. 
      305     Old : New UE, original (primitive) colour reconnections
    Note 1: care should be taken when interpreting results obtained with these
    tunes. They have not all been tuned to the same data, and not to the same
    level of sophistication. For more details about each tune / model, see 
    the list of parameters and useful references printed by PYTUNE for
    each specific model. (This printout can be switched off with MSTU(13)=0.) 
    Note 2: care has been taken so that these parameter settings should
    also work when used 'in sequence', i.e. when several different models 
    are initialized one after the other during a single Pythia run, but we 
    have not explicitly verified this for all possible combinations. If 
    running multiple models in sequence you are therefore advised first 
    to make a few quick sanity checks first, comparing e.g. <Nch> to what 
    you get when initializing only one model in each Pythia run.
    Note 3: the PYTUNE routine also contains tune parameters for the GAL and
    SCI scenarios of the Uppsala group. These options can only be run with a
    modified version of PYTHIA 6.215, however, as described in the routine.
    If you want to use those, you therefore have to extract PYTUNE and link
    it together with the modified PYTHIA version on the Uppsala web site.
    The GAL and SCI models cannot be used in sequence with other PYTUNE calls.
  - SLHA Decay Tables. Bug fix in PYSLHA. When reading in SLHA decay tables 
    for 3-body decays, erroneous behaviour could occur since Pythia expects 
    a specific ordering in the list of decay products while the SLHA standard 
    does not impose such ordering. There were a few cases (e.g. sparticle in 
    SLHA file appearing as decay product 2, with absolute KF code of number 
    3 larger than that of number 1, or coloured particle appearing as decay
    product 2) where PYSLHA failed to correctly reorder the decay products. 
    This has now been amended.
  - External particles, QNUMBERS, PARTICLE. Improvements in the read-in 
    of external particles. PYSLHA is now more intelligent when forming the 
    name of antiparticles, replacing '+' by '-' etc for charged states, 
    rather than just appending 'bar'. PYSLHA now also deals better with 
    reading mass spectra for completely generic external states. Finally, 
    to test out alternative naming schemes (QNUMBERS may be included in  
    SLHA2 but the standard has not yet been agreed upon), Pythia will 
    understand BLOCK PARTICLE to mean the same thing as BLOCK QNUMBERS.

6.4.09 : 13 November 2006
  - Division in DATA statements in PYALPS removed, owing to problems 
    for some compilers.
  - PYTIME modified so no time information at all by default, again
    owing to problems for some compilers.

6.4.10 : 30 January 2007
  - Bug corrected in initialization of Les Houches colour tags in /PYCTAG/. 
    The number of colour tags, NCT, was not consistently reset to zero 
    for each new event, leading to a possible slowing-down and eventually 
    crashing of the program for certain specific settings, e.g. for 
    Tune ACR (using the PYTUNE preset 107). NCT is now explicitly reset 
    to zero at the beginning of each event.
  - Added warning in PYINOM to clarify the treatment of chargino and
    neutralino masses in extreme cases.  When the internal PYTHIA code
    is used to calculate the Supersymmetric neutralino and chargino masses,
    it is prohibited for the lightest chargino to be lighter than the
    lightest neutralino.  When this would otherwise happen, the lightest
    chargino mass is forced to be larger than the lightest neutralino mass 
    plus two times the pion mass.  This choice is based on the fact that 
    gauge boson loop corrections dominate over Supersymmetric corrections 
    for many cases (see Nucl. Phys. B543: 47-72,1999 for a discussion).
    To avoid  confusion about this, an explicit warning is now printed whenever 
    this happens. We stress that the calculation of sparticle masses
    and mixing angles from input soft Supersymmetry breaking parameters
    is (for the most part) treated at only the tree level in PYTHIA.   
    Currently, a large number of dedicated (external) codes exist to
    calculate the Supersymmetric spectrum at higher orders.  These
    can be interface with PYTHIA using the SLHA interface.  With these tools,
    the interested user can study, for example, the case of chargino decays 
    to a single pion or a quasi-stable chargino. 
  - An additional warning statement is added to PYAPPS to remind the
    user whenever highly approximate formulae are used to calculate
    the Supersymmetric particle spectrum.  This code was and is only
    intended for debugging purposes.
  - Change in PYSLHA, for read-in of particle masses via the SLHA.
    Earlier, PYSLHA would only read the mass given in BLOCK MASS 
    for particles either unknown to PYTHIA or relevant to the (N)MSSM. 
    Now, the mass of any particle can be read from BLOCK MASS. For 
    instance the mass of a Z' (KF=32) could not previously be read in
    via SLHA. This is now possible.
  - Change in PYSLHA, for read-in of BSM states via the SLHA.
    The introduction of new states via the proposed SLHA
    BLOCK QNUMBERS (or, alternatively, BLOCK PARTICLE) has been made
    more general and especially particle names are formed in a more 
    stable way.
  - Changes in PYSLHA, for read-in of decay tables via the SLHA.
    1) Behaviour made more stable against inconsistent inputs.
    DECAY statements without accompanying explicit decay channels 
    are ignored, unless the WIDTH is zero, in which case the particle 
    in question is set stable. Previously, if a DECAY line contained 
    a non-zero width for a particle but no explicit decay channels 
    were given, errors would be printed and program execution adversely 
    affected. 
    2) For a known resonance, DECAY entries are now simply placed
    at the end of the internal decay table, and MDCY(KC,2) and 
    MDCY(KC,3) updated accordingly. Previously, the existing decays 
    were first removed and all other particle decays moved down. This
    had the advantage of optimizing memory usage, but also meant that the 
    decay channels of all particles following the modified one were 
    renumbered, which could lead to problems for some applications. 
  - An event that fills up and tries to overflow the whole PYJETS 
    commonblock can enter a loop (possibly at times infinite) where many 
    unsuccessful tries are made, especially for the new PYEVNW machinery.
    Therefore both PYEVNW and PYEVNT have been modified so that such 
    overflow problems leads to the current event being thrown away and 
    a new one generated. 
 
6.4.11 : 30 March 2007
  - Further changes in PYSLHA, for read-in of decay tables via the
    SLHA. Decay modes which contribute to the total width but which are
    not desired active for the purpose of the present run
    can now be switched off simply by commenting out (or deleting)
    the corresponding line in the SLHA file.
    PYTHIA will then add an inactive dummy mode containing the sum of
    switched-off branching fractions, such that cross sections etc
    will scale correctly as the effective branching fraction
    decreases.
    Previously, PYTHIA would force a rescaling so that the
    sum of active branching fractions would be equal to unity - that
    rescaling is now only applied if the sum of active branching
    fractions is larger than unity.
    Note that, alternatively, the same thing can be done by giving a
    negative sign to the branching fraction for the mode(s) to be
    switched off. Such a mode will then still appear explicitly in
    PYTHIA's decay listing (with a branching fraction equal to the
    absolute value of the one in the file), but it will be switched off
    for the event generation run.
  - PYSLHA: Eliminated leading space in particle names when reading in 
    QNUMBERS/PARTICLE blocks
  - Update in PYSGSU. New parameter RMSS(42) allows possibility to set
    the SUSY-QCD Yukawa (quark-squark-gluino) coupling to be different
    from the corresponding gauge (quark-quark-gluon) coupling. In softly 
    broken supersymmetry, these two couplings must be identical (at tree 
    level), but it can be phenomenologically interesting (see e.g. 
    hep-ph/0703160) to investigate the consequences of allowing them to be 
    different. The default value of RMSS(42) has been set to 1, which
    corresponds to the ordinary SUSY conserving relation. If the user
    sets RMSS(42) different from unity, the following relation obtains:
    alpha_s' = RMSS(42) alpha_s  ,
    where alpha_s' is the coupling of the squark-quark-gluino vertex 
    (squared), and alpha_s is the QCD gauge coupling (squared).
  - New routine PYONOF, callable directly or more commonly called
    indirectly from PYGIVE, to allow decay channels to be switched
    on and off based on their decay-product content. Thereby the
    more error-prone addressing by absolute decay-channel number
    can be avoided.
    When called, the routine loops through all the decay channels
    of the intended particle, checks whether each matches the
    criteria, and then individually switches them on or off. It
    only works for simple on/off cases, however, see notes below.
    The argument to PYONOF is a CHARACTER string of the format
        KF:PROPERTY = LIST
    Here
    * KF is the PDG code of the decaying particle
      PROPERTY tells which action to take
        = ALLON, ALLOFF means that all decay channels are  switched
            on or off; expects no list (or equal sign).
        = ONIFANY, OFFIFANY means that a decay channel is switched
            on or off if one of its decay products matches one of
            the particles in the list. 
         = ONIFALL, OFFIFALL means that a decay channel is switched
            on or off if each of the particles in the list is matched
            to one particle in the channel. The channel may well
            contain more particles than the list, but cannot contain
            fewer. If the same code is repeated in the list then it
            must also occur (at least) the same number of times in
            the decay channel.   
         = ONIFMATCH, OFFIFMATCH works as ONIFALL, OFFIFALL, with the
            difference that there must be a perfect match, i.e. the
            number of particles in the channel and in the list must
            agree.
      LIST is a list of PDG particle codes, at most 10 of them,
            with the particles separated either by blanks or by
            commas. It is expected that all codes are positive, and
            are compared with the absolute value of the products
            in the decay channel.
    Examples:
        CALL PYONOF("23:alloff")
        CALL PYONOF("23:onifany = 1 2 3 4 5")
    first switches off all Z0 decay and then switches back on those
    to quarks.
        CALL PYONOF("15:offifany = 311 321 310 130 313 323")
        CALL PYONOF("15:onifmatch = 16 323")
        CALL PYONOF("15:offifall = 211 211")
    first switches off all decay modes of the tau that contains a K
    or K* meson, then switches back on the specific nu_tau K*- channel,
    and finally switches off all channels with at least two charged
    pions.
    Note 1: the method only works to set values MDME(IDC,1) = 0 or 1,
        corresponding to off or on. Non-existing channels, i.e. those
        with negative MDME(IDC,1) values, are never changed. Values
        above unity, used e.g. to denote different behaviour for a
        particle and its antiparticle, cannot be set by this routine,
        but existing such values will be changed by the commands above.    
    Note 2: the whole string cannot be longer than 100 characters.
    Note 3: an exclamation mark (!) can be used to denote the
        beginning of a comment.
    Note 4: the PROPERTY can be written in upper- or lowercase,
        or mixed, as desired.
    Note 5: the routine echoes back the command it believes it has
        been told to execute, and prints a warning if it failed to
        interpret it.
    Note 6: obviously the final outcome depends on the order in which
        commands are carried out. Always use CALL PYLIST(12) to check
        that you got it right. Recall that MSTU(1) and MSTU(2) can be
        used to restrict the range of KF codes that are actually listed.
  - PYGIVE has been modified to that if the input string begins with
    a digit the command is sent on to PYONOF for processing there,
    as described above.
  - Update to PYLOGO to reflect current author bio information.
  - Bug fix in PYSGSU for the treatment of Supersymmetry process 276 (the
    production of q_L q_Rbar or q_Lbar q_R).  Previously, when branching
    fractions were controlled by the user, the overall rate was reduced
    as if a q_R q_Rbar pair was produced.
  - Bug fix in PYTBDY to correct a problem where an undefined particle code 
    would sometimes arise in the decay of a gaugino to a gaugino and a
    fermion-antifermion pair.
 
6.4.12 : 23 July 2007
  - Update of PYTUNE. The value of the switch MSTP(68) is now included
    in the printed list of parameters, together with a statement that
    this parameter is not being (re-)set by PYTUNE. The reason for
    including this parameter is that the shower phase space for
    processes without ME matching and which contain no QCD jets is
    affected by it. Since no such processes were included in tuning
    exercises so far, its value is not fixed, hence PYTUNE leaves it
    at the current default, MSTP(68)=3 ('power showers').
  - Correction in PYKLIM for the case when cuts on pTHat (CKIN(3), CKIN(4))
    and asymmetric cuts on cos(thetaHat) (CKIN(27), CKIN(28))
    conspire to close one of the two regions with cos(thetaHat)
    > 0 and < 0, but not both.
  - PYMEWT expanded to include a matrix element correction to the parton
    shower in PYSSPA for processes of the type f fbar -> h0 (A0, H0),
    similar to the corrections made for W and Z boson production.
    This is mainly of interest for the production of Higgs bosons that
    have an enhanced coupling to heavy quarks, and ensures that b bbar
    -> h0 matches smoothly onto b g -> b h0 + h.c.  This includes
    processes ISUB=3, 151, and 156. 
  - PYSIGH modified to treat processes described above (3, 151, and 156)
    with ISMECR=1.
  - In version 6.410 a bug was corrected in PYEVNT for Tune ACR [PYTUNE(107)] 
    concerning resetting the number of Les Houches colour tags for each new 
    event. However, the common block /PYCTAG/ was not declared in the header 
    of PYEVNT. This is now remedied.
  - MSTP(98) (D=0): treatment of beam remnant in PYSPLI when a photon is 
    extracted from an incoming baryon beam.
     = 0 : the baryon is split up into a quark and a diquark as usual.
     = 1 : the baryon is assumed to radiate the photon coherently, 
            i.e. without breaking up. 
  - PYSTOP subroutine added to handle all instances when, previously, a
    Fortran STOP occurred in the code.  The default behavior of PYSTOP is to
    execute a Fortran STOP, but the user may modify this routine.
  - PYVETO can now be called for pT ordered showers.  
  - MSTP(152) (D=0): treatment of final state radiation for the additional
    interactions produced in the old multiple interaction model.
    = 0 : no showering
    = 1 : PYSHOW is called with the scale SQRT(PARP(71)) times the pT of the
           interaction in PYMULT.
  - PYEVNW modified to correct a problem with color tags for external events of
    the type e+e -> multiple colored partons.  Also, the time sequence
    for showering external events is changed to follow the treatment in PYEVNT.
    This means that showering parameter PARJ(81) is now set to PARP(72) before 
    performing final state radiation on external events. 
  - PYRESD modified to handle a problem with a particle mother of 0
    in external events producing colored particles with multi-body decays 
    which also parton shower.
  - Problem with negative KF codes fixed in PYTBDY, affecting (slightly) three
    body decays of sparticles.  This problem was partially fixed in 6.411.
  - Correction of showering options in PYSHOW available for colour-octet
    charmonium and bottomonium production, as regulated by the
    MSTP(148) and MSTP(149) switches. In particular, MSTP(148) = 0
    did not switch off radiation as claimed, but allowed it with
    a reduced colour factor, as for q -> qg, as opposed to the normal
    g -> gg one. This has now been corrected, and MSTP(149)=-1 has
    been added as a new option to allow backwards compatibility.
    Furthermore the description of the MSTP(149)=0 option was not
    quite correct and is clarified below. The options now read:
    MSTP(148) (D=0) : radiation off onium states.
        = 0 : off.
        = 1 : on.
    MSTP(149) (D=0) : form for this radiation when MSTP(148) = 1.
        = 0 : use the same splitting description as for g -> gg
            (corrected by mass effects) but trace the
            "on the average" harder gluon by letting
            1/(z(1-z)) = 1/z + 1/(1-z) -> 2/(1-z)
            in the splitting kernel.
        = 1 : use the same splitting description as for g -> gg
            (corrected by mass effects), i.e. allow the onium to
            take a smaller fraction in the splitting than with = 0.
        = -1 : as = 0 but use a smaller colour factor as for
            q -> qg.
        Note: it is not clear what is the "correct" behaviour.
           One reasonable approach to explore uncertainties would
           be to view MSTP(148)=1, MSTP(149)=0 as a central value
           and MSTP(149)=+-1 as two outliers.
  - Typo found in the Pythia 6.4 manual: in section 8.5.3 towards
    the end it should read that RMSS(11) sets the right sbottom
    mass parameter and RMSS(12) the right stop one, not the
    reverse. The parameter listing in section 9.5 is correct. 
  - Changes to PYCTTR to make it more stable against being called to trace 
    "garbage" particles, i.e. entries with K(I,1)<=0 or PYCOMP(K(I,2))=0. 
    In such cases the routine now just returns without doing anything.
  - PYPTFS: removed unsafe colour tracing which produced crashes with the 
    new models, e.g. PYTUNE(300).  Made the determination of IANNFL safer.
    Corrected a spelling mistake and introduced zeroing of irrelevant MCT 
    tags in addition the KCS ones. 
  - Handle case of small negative partial widths in PYH2XX.
  - Changes to PYPTIS to prevent crash for lepton beams.
  - Minor bug fix in PYSLHA to prevent out-of-range problems for CPRO and CVER.

6.4.13 : 12 September 2007
  - Bug corrected for processes 174 and 179, WW vector boson fusion
    of a H0 or A0, respectively, where the H0 is the heaviest, CP
    even Higgs boson and A0 is a CP odd Higgs boson in an extended
    Higgs sector. Incorrectly the Z mass instead of
    the W mass was used to describe the kinematics, giving too small
    a cross section by about a factor 1.5 - 2. Process 124, the
    corresponding process for h0, the Standard Model Higgs boson or 
    the lightest, neutral CP event Higgs boson in an extended model,
    production, is not affected.
  - PYTUNE updated with three new tunes using CTEQ6: 
        108 D6  : Rick Field's CDF Tune D6 
        109 D6T : Rick Field's CDF Tune D6T 
        306 ATLAS-CSC: Arthur Moraes' (new) ATLAS tune
    Note: PYTUNE can now also be called automatically during PYINIT by
      setting MSTP(5). See separate comments on introduction of
      MSTP(5) also in this version
    Note 2: since the CTEQ6 family is not included with the Pythia
      code, they must be interfaced manually for these tunes to give
      proper results. See MSTP(51) and MSTP(52).
    Note 3: more elaborate comments are included inline in PYTUNE 
  - New switch MSTP(5) introduced to set UE/ISR/FSR 'tune'. If MSTP(5)
    is set non-zero by the user, the routine PYTUNE will automatically
    be called during PYINIT with the tune number indicated by the
    value of MSTP(5).
    MSTP(5) : (D=0) specification of global parameter set, if any, for
       underlying event, min-bias, bremsstrahlung (ISR and FSR), beam
       remnants, primordial kT, etc. Current available 'tunes' are:
        = 0 Default : No settings changed.
        = 100 A   : Rick Field's CDF Tune A
        = 101 AW  : Rick Field's CDF Tune AW
        = 102 BW  : Rick Field's CDF Tune BW
        = 103 DW  : Rick Field's CDF Tune DW
        = 104 DWT : DW with alternative (default) UE energy scaling
        = 105 QW  : Rick Field's CDF Tune QW 
                    Note! needs CTEQ6.1M pdfs externally
        = 106 ATLAS-DC2: Old ATLAS tune (ATLAS DC2 / Rome)
        = 107 ACR : Tune A modified with annealing CR
        = 108 D6  : Rick Field's CDF Tune D6 
                    Note! needs CTEQ6L pdfs externally
        = 109 D6T : Rick Field's CDF Tune D6T 
                    Note! needs CTEQ6L pdfs externally
        = 200 IM1 : Intermediate model: new UE with Q2-ordered showers,
        = 201 APT : Tune A modified to use pT-ordered final-state showers
        = 300 S0  : Sandhoff-Skands Tune 0
        = 301 S1  : Sandhoff-Skands Tune 1
        = 302 S2  : Sandhoff-Skands Tune 2
        = 303 S0A : S0 with "Tune A" UE energy scaling
        = 304 NOCR: New UE "best try" without colour reconnections
        = 305 Old : New UE, original (primitive) colour reconnections
        = 306 ATLAS-CSC: Arthur Moraes' (new) ATLAS tune
                         Note! needs CTEQ6L pdfs externally
    Note: for the present, the call to PYTUNE is placed before the
       call to UPINIT, so setting MSTP(5) as part of UPINIT will not
       work. 
  - Note on 'power' vs 'wimpy' showers: applies only to processes for 
    which ME corrections are not implemented and which do not have any jets 
    in the hard matrix element, i.e., most notably including ttbar and SUSY 
    processes, but not e.g., Drell-Yan (has matrix-element corrections) or 
    min-bias (has QCD jets).  Regulated by MSTP(68). In Pythia 6.2, the 
    default was MSTP(68)=1  whereas it is currently MSTP(68)=3. Since no data 
    used for tuning has so far been sensitive to this difference, PYTUNE does 
    not explicitly (re)set this switch, even when selecting tunes that were 
    originally done with 6.2, such as Tune A and friends.  If desiring to 
    reproduce 6.2 results for all processes, the user should therefore 
    manually set MSTP(68)=1. A warning about this has been included. 
  - Parameters MSTU(1) and MSTU(2), which control the particles which
    are rotated, boosted, edited or listed by Pythia routines, are 
    now zeroed in PYEVNT and PYEVNW as an extra protection.
  - Corrected error where the decay of a heavy Higgs boson H0 -> Z0 + A0 
    was disabled.
  - Two revisions of how information is stored to the event record by the 
    pT-ordered, initial-state shower, PYPTIS. Should not affect physical 
    behaviour. In the documentation lines, the incoming partons 5&6 are 
    now left unmodified relative to the initial hard scattering, apart 
    from the collective boosts and rotations received by the hard system 
    as a consequence of initial-state radiation. The hard system final 
    state(s), line(s) 7-... remain as before. This has the virtue of keeping 
    the entire hard system unchanged, apart from an overall boost+rotation. 
    In particular, 4-momentum is explicitly (and locally) conserved across 
    the hard interaction and partons 5 & 6 remain on shell. In reality, 
    of course, the initiator partons receive spacelike virtualities in the 
    cascade. These are now kept track of during the parton evolution and so 
    spacelike partons (denoted by P(I,5) negative) will now appear in the 
    non-documentation part further down in the event record. Partons 5 and 6 
    thus no longer correspond directly to any of the partons appearing in the 
    non-documentation lines. 
  - Added checks in PYTBDY to make sure that non-SUSY, three-body, resonance 
    decays are treated separately.  Three-body decays of a resonance, added 
    through the SLHA mechanism, for example, will be decayed according to
    phase space, whereas the internally-defined decays of SUSY particles
    are weighted by the squared matrix element.
  - PYLOGO and PYDATA comments: added new CERN address for Skands.

6.4.14 : 09 November 2007
  - Bug corrected in PYRESD (already in 6.413, but the corresponding
    update note was left out). Added check for Higgs -> ZZ/WW before
    applying angular correlations 
  - Bug corrected in PYPTFS. Affecting matrix-element matching for
    pT-ordered final-state showers. After a trial branching had been
    vetoed by the matching step, the evolution was restarted from the
    original scale, rather than from the scale of the vetoed
    branching, effectively nullifying the matching. The evolution is
    now restarted from the vetoed scale, which should give the correct
    matching. The previous recommended value of PARJ(81)=0.14 for
    these showers was based on comparisons to LEP; this should now be
    changed to roughly 0.23 (to be compared with 0.29 for the old
    mass-order shower). Thanks to Nils Lavesson and Leif Lonnblad for
    finding this bug.
  - Update of PYTUNE. Affecting all tunes using pT-ordered final-state
    showers. Lambda_QCD for final-state showers (PARJ(81)) changed
    from 0.14 to 0.23 for tunes 201 and 300-306 (including the
    ATLAS-CSC tune by A. Moraes). This change is necessary due to the
    bug fix in PYPTFS. The previous Lambda_QCD value had been tuned
    using the buggy shower (see above).
  - Bug corrected in PYMEWT. Affecting the matching of initial-state
    showers in the process q qbar -> V (V = W+- or gamma*/Z0) to the q
    g -> q V matrix elements: a forgotten swap of tHat and uHat in the
    numerator of the matrix element. This correction reduces the mean
    pT of the gauge boson by order 10%, other things being the
    same. It would require a retune to restore the old behaviour,
    which has not been done so far.
  - Bug corrected in PYMIRM. Affecting transverse phi coordinate of
    scattering subsystems when using the new beam remnant
    treatment. Previously, an unwanted rotation in phi was applied to
    each scattering subsystem during the assignment of primordial
    kT. The phi rotation has now been corrected, so that it goes
    correctly to zero in the limit of small primordial kT. Thanks to
    Johan Alwall for finding this bug.
  - Added safety check in PYRAND. Affecting external processes. PYRAND
    now explicitly checks the x fractions in external processes and
    prints a warning together with an event listing if they are
    greater than unity.
  - Update in PYPTIS. Affecting error messages from PYPTIS, the
    pT-ordered initial-state parton shower. The error reporting from
    PYPTIS has been made slightly less fussy, with weighting problems
    for low-pT radiation relegated to warnings rather than errors.
  - Substantial updates of PYSLHA. Introduction of LHEF
    interface. PYSLHA is now able to read QNUMBERS for new BSM states,
    MASS entries, complete SLHA1 spectra, and SLHA DECAY tables
    directly from the header of the LHEF file. Moreover, it looks for
    this information automatically, without the necessity of
    explicitly setting the usual IMSS switches. 
  - Further generalizations of PYSLHA. Affecting functionality when
    PYSLHA is used stand-alone to read in QNUMBERS, MASS, and DECAY
    tables. Setting the second argument to PYSLHA equal to zero now
    has the effect that PYSLHA reads everything in the file, e.g.:
      CALL PYSLHA(0,0,IFAIL) ! read all BLOCK QNUMBERS
      CALL PYSLHA(5,0,IFAIL) ! read all entries in BLOCK MASS
      CALL PYSLHA(2,0,IFAIL) ! read all DECAY tables. 
  - Update in PYMSIN. Changes to automate the new SLHA/LHEF interface.
  - Update in PYRESD. A more robust algorithm to set color flows in
    resonance decays has been introduced, to improve the capacity to
    treat generic 2- and 3-body resonance decays, e.g., in conjunction
    with the new SLHA/LHEF interface.
  - Strawman Technicolor Model updated to include the effects
    of a techni-"a" meson a_tc, the analog of the a0(980) from the PDG.  By 
    convention, the particle code for the neutral and charged a_tc are
    3000115 and 3000215, respectively.  In accord with much of the technicolor
    simulation, the a_tc does not appear as a resonance in the particle record,
    but its effect is included in the differential cross section formulae for
    the production of a gauge boson in association with a technipion.  
    Interference with rho_tc and omega_tc is included, where appropriate.
    Five new parameters are introduced to specify the effective Lagrangian:
    RTCM(47): (D=1.0) the ratio of the coupling g_a-pi-pi to g_rho-pi-pi 
      in technicolor
    RTCM(48-49): (D=200,200) vector and axial mass parameters (in GeV) for 
      the a-G-pi interaction, where G is a SM gauge boson
    RTCM(50-51): (D=200,200) vector and axial mass parameters (in GeV) for
      the a-G-rho interaction, where rho includes the techni-rho and -omega
    The masses of a_tc0 and a_tc+/- can be set through
    PMAS(368,1) and PMAS(369,1), respectively.  The total width and
    branching ratios are calculated by default.

    The addition of the techni-a meson modifies most of the technicolor
    processes (194,195,361-377), and requires the addition of three new processes:
    378 (W+gamma), 379 (Z+gamma) and 380 (Z+Z).  Because of an ambiguity
    in defining the connection between goldstone bosons and longitudinal
    gauge bosons, the processes mix combinations of longitudinal and 
    transverse polarizations.   All gauge bosons produced in the technicolor
    simulation are decayed without spin information, as before.
    Several routines used to calculate eigenvalues of propagator matrices have 
    also been modified to account for the techni-a.  The sampling of kinematic
    variables has also been improved.  The specific changes to the code are 
    the following:
    PYTECM is used to find the actual pole and width of the propagator matrix
     for processes including gamma/Z/rho_tc0/omega_tc0/a_tc0 and W/rho_tc+-/a_tc+-.
    PYSIGH, PYWIDT, and PYDATA are updated to include the techni-a meson in cross 
     sections, decay widths, and to set default parameter values, respectively.
    PYWIDX, and auxiliary routine for techni-particle width calculations, was 
     reduced in size.
    PYEICG, PYCBA2, PYCBAL, PYCMQ2, and PYCRTH are modified to find the 
     eigenvalues of a NxN matrix, with N<=5.
    PYMAXI, PYRAND, PYKMAP, and PYSIGH are modified in two ways:
     1. To find more accurately the actual poles in technicolor processes and
      use those for selecting the tau variable.
     2. To allow a third Breit-Wigner resonance in the selection of the tau
      variable.
   -PYXXZ6, and auxiliary routine for calculating Supersymmetric decay widths,
     was corrected so that, if a negative weight is found, the resulting 
     warning message respects the bounds of an array.  This otherwise harmless
     mistake can cause the code to abort with certain compiler options.
   -PYINOM had a true bug fix for the calculation of the U and V matrices
    which diagonalize the chargino mass matrix.  Since U and V are calculated
    using the squared mass matrix, it is possible to miss a diagonal phase,
    which can be re-absorbed into the definition of V.  Earlier versions of
    PYINOM did not suffer from this problem.  Regardless, it appears that
    only 3-body decays of charginos involving the third generation are 
    sensitive to this error, and then at a several percent level. 

6.4.15 : 25 February 2008
  - Significant speed-up of PYFSCR algorithms. Affecting tunes S0 (300), S1
    (301), S2 (302), S0A (303), ACR (107), and in general any model using the
    new colour-annealing models for colour reconnections. Inspired by
    FastJet, the annealing algorithm has been rewritten and substantially
    optimized. The speed gain is most noticeable at the LHC. For typical
    processes on a standard PC the speed gain is (for PYFSCR stand-alone
    (approximately) and for generation of a complete event, respectively): 
            Speed Gain             PYFSCR      Complete Event
            -------------------------------------------------
            ttbar @ LHC :         x 400.0          x 6.0
            min-bias @ LHC  :     x 150.0          x 3.0
            ttbar @ Tevatron :    x 100.0          x 2.5
            min-bias @ Tevatron : x  20.0          x 1.3
  - Bug corrected in PYFSCR, affecting Tune ACR (PYTUNE(107), see below) and
    in general any tune that uses the old (non-interleaved)
    multiple-interactions framework together with the new colour-annealing
    model. Previously, the suppression of reconnections inside the same beam
    remnant was incorrectly suppressing reconnections among UE partons from
    the old model outside the beam remnant as well. 
  - Update of Tune ACR (PYTUNE(107)), following the bug fix in PYFSCR
    mentioned above. After the bug fix, the algorithm now allows more
    reconnections when using the old MI model with the new CR algorithm, so
    the overall strength of the color-reconnection parameter has been tuned
    down to compensate; PARP(78) changed from to 0.25 to 0.12 for Tune
    ACR. This retuning only takes into account <Nch> at generator-level in
    min-bias at the Tevatron. Further retuning could be needed to restore
    agreement with other distributions, such as <pT>(Nch).  
  - Bug fix in PYSLHA, concerning the SLHA+LHEF interface for BSM
    generators introduced in 6.4.14 and described in
                    http://arxiv.org/abs/0712.3311
    During SLHA read-in from an LHEF file, PYSLHA would continue down
    to the first <event> tag. Between </slha> and <event>, however, it
    risked crashing due to attempting SLHA read-in of non-SLHA
    information. PYSLHA now exits when either </slha>, <init>, or the
    first <event> tag is reached. Thanks to A. Belyaev for reporting
    this.
  - Bug corrected in PYMIRM, concerning the treatment of beam remnants with 
    small invariant masses in the new interleaved framework. Previously, it 
    was possible to find a solution in which each of the remnants separately 
    had negative masses, but where their combination was timelike, 
    corresponding to both of the beam remnants moving "in the wrong 
    direction". Due to the lightcone nature of the calculations involved, 
    this led to beam-remnant partons with negative energies and causing 
    violations of energy and momentum conservation in the final events. 
    A check has now been added requiring the sum of the individual beam 
    remnant masses to be positive. Thanks to B. Kersevan for pointing to 
    this problem.
  - PYPTFS corrected for uninitialized IORD variable when matrix
    element corrections switched off.
  - Protect against potential division by zero in PYCLUS.
  - String length corrected in PYDATA.
  - Expanded usefulness of the Z' and W' to represent generic neutral
    or charged resonances, when using the MWID(32)=2 and MWID(34)=2
    options. Additionally, for Z', one must set MSTP(44)=3 so that
    the contributions from gamma*/Z0 are off. You can set your own
    total width and branching ratios for the respective decay channels,
    and this information will then be used both to select decays and
    to deduce the couplings to the different incoming beam partons.
    Since the branching-ratio information does not fix the correct
    angular distributions, the Z' and W' will decay isotropically
    in these options. This could subsequently be reweighted by the
    user, if required. 
  - PYDATA and PYLOGO updated to reflect that it is now 2008.

6.4.16 : 07 March 2008
  - Correction to Manual for SUSY with RPV. In the current
    documentation, it is claimed that when you initialize RPV with
    IMSS(51)=1 for LLE, IMSS(52)=1 for LQD, and/or IMSS(53)=1 for UDD,  
    Pythia sets all the couplings, independent of generation, to a common
    value of 10^{-RMSS(51)}, 10^{-RMSS(52)}, and/or 10^{-RMSS(53)}, 
    depending on which couplings are activated. In the code, however, 
    the couplings are just set to RMSS(51), RMSS(52), and/or RMSS(53),
    respectively, i.e., without taking the negative power of
    ten. Thanks to K. Matchev for pointing out this discrepancy. 
  - Revised PYSGHG routine for calculating gg->h,H,A cross sections as well as 
    the equivalent gammagamma cross sections. Affects processes 
      102,152,157: g + g -> h0/H0/A0
      103,153,158: gamma + gamma -> h0/H0/A0
      111,183,188: f + fbar -> g + h0/H0/A0 
      112,184,189: f + g -> f + h0/H0/A0 
      113,185,190: g + g -> g + h0/H0/A0
    The h,H,A->gg/gammagamma branching ratio, which enters the calculation 
    of the corresponding cross sections by crossing symmetry, had been 
    fixed since v.6.402 to be the branching fraction evaluated at the 
    pole mass of the respective Higgs. The pre-6.402 behaviour, computing 
    the effective branching ratio at the actual mass scale of the reaction, 
    has now been restored, with the fixed-width alternative only used 
    in conjunction with SLHA decay tables. The main effect is to avoid 
    an undesirable peaking of the cross section towards small Higgs masses, 
    where the gluon density sharply increases, but where the effective 
    branching fraction simultaneously should go to zero, due to the gg->H 
    coupling vanishing for small m-hat.  
  - In PYSGHF, for process 83, set width suppression factor correctly.
  - In PYMAXI, modified the algorithm for solving a set of linear equations 
    when optimizing the selection of phase space.   The current
    implementation is less susceptible to round-off error, and does
    not show a sensitivity to compiler optimization.

6.4.17 : 04 June 2008 
  - Introduction of UED spectrum, cross sections, and decays. See
    detailed update notes at the beginning of this file. 
  - Update of PYTUNE, affecting primordial kT for tunes 300-305 
    (S0,S1,S2,S0A,NOCR,Old). The width of the primordial kT distribution is 
    no longer set equal to a constant, but varies with the Q2 of the hard 
    interaction according to the parametrization described in section 4.3 of 
    hep-ph/0402078. This prevents the large primordial kT value needed 
    in Drell-Yan to give large enhancements to the pT distributions in 
    low-Q processes, such as minimum-bias. 
  - Modified PYINOM. Changed message when forcing 
    m(~chi+_1) > m(~chi0_1) + 2m(pi0) to give a warning instead of an error.
  - NOTE: THIS VERSION REVOKED DUE TO COMPILATION PROBLEMS, SEE 6.4.18.

6.4.18 : 09 June 2008
  - Changes to address compilation problems in 6.4.17, as follows:
  - Revised PYXDIN. Added missing IMPLICIT DOUBLE PRECISION statement.
  - Corrected definition of PYSGCM common block in PYXUED.
  - Corrected minor syntax problems in PYDATA and PYUEDC.
 
6.4.19 : 21 October 2008
  - Update of PYVETO. Due to modifications introduced in 6.4.12, the
    interface to AlpGen was effectively broken. The order of the event 
    record entries used in PYVETO has now been restored to that expected
    by AlpGen. This applies to both the Q2- and pT2-ordered
    showers. Matching with AlpGen should therefore now be working again.
  - Bug fix in PYRESD, affecting the behaviour for pT-ordered
    ISR. In the new shower, the incoming partons after ISR are stored
    in a way which may result in inconsistent boosts when smearing 
    decay angles in post-ISR resonance decays. However, the
    corresponding partons in the documentation section (partons 5 and
    6) have consistent on-shell momenta that can be used to derive
    these boosts instead. Ultimately, the way the new shower stores
    intermediate partons should be changed, but this would require a
    major rewrite. For the time being, a provisional solution using
    partons (5,6) to define the boost has therefore been
    adopted. Thanks to Serguei Levonian for pointing to this problem.
  - Changes in PYPTIS to introduce new coherence options for the first
    emission in pT-ordered ISR. Previously, MSTP(67) only applied to
    the old Q-ordered ISR showers. Now, the options described below
    have been implemented for the pT-ordered showers. (The meaning of
    MSTP(67) for the old showers, described in the manual, has not been
    changed.) The changes are most significant for processes that
    were previously "power showered" (that is, internal processes
    without ME matching and no jets in the matrix element). The
    default, which remains MSTP(67)=2, now corresponds to
    "power-suppressed power showers" for those processes, since the
    tail of very high-pT radiation becomes suppressed by
    coherence. 
    MSTP(67) = 0 : No coherence imposed (equivalent to pre-6.4.19
                   behaviour). 
             = 1 : Full coherence imposed. As =0, but a veto is
	           imposed on the first ISR branching to impose 
                            pTevol < m_dip/2 * PARP(67) , 
                   where m_dip is the invariant mass of the ISR parton
		   being evolved together with its final-state color
		   partner (or the initial-state parton in the other
		   beam, for annihilation color flows). For gluons, a
		   random choice is made between its two color
		   partners. Imposing this "coherence veto" should
		   often give similar answers as wimpy showers, since
		   the latter effectively veto emissions above the 
		   factorization scale, and m_dip should often be
		   close to that. 
             = 2 : Hybrid (default). As =0, but ISR branchings with
	           pTevol > m_dip/2 * PARP(67) are power suppressed,
		   by a factor (m_dip/(2pTevol))**2. This
		   automatically suppresses branchings with pT values
		   far above the coherence scale, while still not
		   leaving that part of phase space completely empty.
    Note: PARP(67) was not previously used for anything in the new
    shower, whereas in the old shower it was used to multiply the
    starting scale of the shower by a fixed factor. As described
    above, we now include this possibility to vary the upper scale of
    the shower also here, allowing the coherence scale to be moved up
    or down by PARP(67). The overall effect of PARP(67) in the new
    shower should thus be similar to that of PARP(67) in the old
    shower. The default value for PARP(67) is currently 4D0 (its tune
    A value), such that coherence will not currently be very strictly
    enforced by default, but the high-pT tail should still be slightly
    more damped than for "pure power showers".
  - Changes in PYPTIS to introduce new option to replace Lambda_MSbar
    by Lambda_MC (see Catani, Marchesini, and Webber, NPB349(1991)635) 
    in the pT-ordered ISR shower, via the switch MSTP(64): 
    MSTP(64) = 1 : (not used for pT-ordered shower, equivalent to = 2)
             = 2 : (Def) Use Lambda_MSbar for ISR evolution
             = 3 : Use Lambda_MC for ISR evolution
    Specifically, the CMW choice of Lambda is obtained by multiplying
    the MSbar Lambda values by the following factors
          Lambda_5^CMW = 1.569 * Lambda_5^MSbar ,
          Lambda_4^CMW = 1.618 * Lambda_4^MSbar , 
          Lambda_3^CMW = 1.661 * Lambda_3^MSbar ,
    where the subscript on Lambda refers to the number of active
    flavors (matched across thresholds in the evolution). The larger
    values of Lambda implies a larger rate of shower activity,
    with the largest increase at low scales, where the number of
    active flavors is smallest. 
  - New option MSTP(70)=3 in PYEVOL to extend the pT-ordered ISR down
    to Lambda_QCD, regulated by an effective alpha_s, whose desired
    value at Lambda should be given in PARP(73). This regulation is
    implemented as an offset to the argument of alpha_s, shifting it
    so that it attains the value PARP(73) at Lambda instead of going
    to infinity.
  - New option MSTP(95)=8 in PYFSCR for colour reconnections. Works as
    MSTP(95)=6, but gluons contribute only half of their momenta in
    the calculation of the Lambda measure, since gluons participate in
    two string pieces. This slightly changes the relative Lambda
    measures of string pieces ending on quarks versus those ending
    on gluons.
  - Bug fix in PYPTFS for processes MSUB=421-479. Prevents crashes for 
    colour octet onium production with the new pT-ordered showers. 
    Note that the non-default option to add showers off certain octet onia - 
    see MSTP(148) - is still only available in the old, Q2-ordered framework. 
    Thanks to Patrick Robbe for pointing to this bug.
  - Bug fix in PYSSPA, affecting the special case when pT-ordered
    final-state showers are used in conjunction with mass-ordered
    initial-state showers (e.g., Tune APT). Previously, PYSSPA would
    already add mass-ordered final-state showers to partons emitted
    during the initial-state shower. This resulted in inconsistencies
    when the pT-ordered shower later attempted to shower the
    event. PYSSPA has now been modified so that, if pT-ordered
    final-state showers are selected (see MSTJ(41)), all timelike
    showering is handled by PYPTFS rather than PYSHOW. In particular,
    this means that the ISR chain is defined with all outgoing partons
    having zero virtuality. These partons are then later showered by
    PYPTFS, using the dipole-style recoils of the pT-ordered shower
    model. 
  - Update of Tune ACR (PYTUNE(107)), following the bug fix in PYFSCR
    mentioned in 6.4.15. The initial retune in 6.4.15 only took into
    account <Nch> at generator-level in min-bias at the Tevatron. The
    <pT>(Nch) distribution was still much too hard, however, so a
    further reduction of the colour reconnection strength PARP(78)
    from 0.12 to 0.09 has now been introduced in this tune.   
  - Update of Tune S0 and cousins. Reverted to the pre-6.4.18
    behaviour for primordial kT, since the change in 6.4.18 was based
    on theoretical prejudice only and actually worsened the agreement
    with some observables, such as the low-Nch tail of <pT>(Nch). It
    remains to be understood why low-multiplicity minimum-bias events
    appear to enjoy getting 2 GeV of primordial kT.
  - Bug fix in PYEVNW. When the initialization of MPI/ISR evolution off
    an event fails, a new event is now generated, rather than the same
    event tried again. This prevents multiple occurrences of the same
    warning from being printed when the evolution repeatedly tries and
    fails to initialize on the same bad event. 
  - Bug fix in PYSLHA, concerning sfermion mixing. The overall sign of
    a row in the sfermion mixing matrices is not physical and hence,
    in the SLHA convention, can be chosen freely. However, Pythia was
    assuming a specific sign convention for these matrices (that the
    diagonal elements have the same sign), which led to erroneous
    answers for sfermion mixing matrices, S, with opposite-sign
    diagonal entries, S(1,1) = - S(2,2). This is now fixed so that
    Pythia correctly translates the input to its internal conventions
    regardless of overall row signs. Thanks to M. Kramer, E. Popenda,
    and P. Zerwas for pointing to this issue and to M. Muehlleitner
    for counterchecks with SDecay. 
  - Bug fix in PYMIHK, for "Lambda" ordering, MSTP(89)=2, with the new
    UE framework. Affects tune NOCR. When using Lambda ordering,
    PYMIHK attempts to compute string length measures for each
    possible arrangement of the MPI scattering subsystems in colour
    space, and then select the arrangement that produces the smallest
    string length. This is not really color *re*-connections, since
    the color lines involved had no connections to begin
    with. Previously, it could happen in intermediate stages of the
    algorithm, that attempts were made to compute string lengths
    involving color lines that had not yet been connected, resulting
    in crashes and/or inconsistencies. Such connections are now
    skipped and returned to later, when all the relevant lines are 
    well-defined. Thanks to Nathan Goldschmidt for finding this. 
  - Relegated two error messages to warnings in PYMIRM: one when kinematics
    makes it impossible to find a positive beam remnant mass squared and the
    other when no consistent (x,kT) sets for beam remnants can be found. Both
    result in a new parton-shower and underlying-event history being 
    generated for the event in question. For most ordinary applications these
    warnings can safely be ignored, as long as they do not occur frequently.
  - Bug fix in PYSTRF for junction fragmentation. The fix prevents crashes 
    that were due to a badly initialized variable. Affects fragmentation 
    of systems containing baryon junctions, i.e., beam-remnant fragmentation 
    in the new underlying-event framework as well as processes involving 
    baryon number violation. Thanks to Amitabh Lath for pointing to this bug.
  - Change in PYEVNW, for MSTP(125)<=1 (D=1), affecting how resonances
    participating in or recoiling against showers are saved in the
    event record in the new multiple-interactions and pT-ordered
    shower framework. Previously, each time a resonance recoiled
    against a shower emission, a new copy of that resonance, with
    modified momentum, would be saved to the event record. This
    resulted in multiple instances of the same resonance being
    present, each with slightly different momenta. For MSTP(125)=2
    (full event records), this is indeed the desired behaviour, but
    for the shortened forms of the event record (MSTP(125)<=1), it is
    more natural to collapse such a chain into one particle, as is
    already done in the old framework. The behaviour of PYEVNW has
    been changed so that the default form of the event record in the
    new framework now more closely resembles that of the old. In both
    cases, the single particle now present in the event record
    corresponds to the resonance _after_ all shower corrections have
    been generated. Thanks to Daniel Wicke for pointing to this issue. 
  - Changes to PYSHOW and PYPTFS. Added new flag MSTJ(39) to switch
    off final-state radiation entirely from a specific particle, with
    PDG code KF = MSTJ(39). The default is MSTJ(39)=0, in which case
    all particles are allowed to radiate as normal. To switch off
    radiation from top quarks entirely, for instance, set MSTJ(39)=6. 
    This is only intended for systematics studies, not to represent
    physics. Applies to both the Q2-ordered and pT-ordered showers.
  - Modifications to PYRVCH, PYRVGL, and PYRVGW, to protect
    against divide-by-zero's. Affects SUSY with R-parity violation. 
  - Bug fix in PYNJDC, corrects a problem in neutralino to gluino 
    decays where some variables could be uninitialized.
  - Minor corrections in PYUEDC. Should not affect physics. Removed a few 
    variables that were declared but not used. Corrected indexing of WDTP 
    (affects initial values only).

6.4.20 : 20 February 2009
  - Comprehensive updates to PYTUNE, with the addition of the
    "Perugia" and "Pro" tunes, following the MPI workshop in Perugia 
    in October 2008. The older tunes remain unaltered. The new
    available tunes in PYTUNE are: 
    ===== Old UE, Q2-ordered showers ====================================
    --- Professor Tunes : 110+ (= 100+ with Professor's tune to LEP) ----
     110   A-Pro : Tune A, with LEP tune from Professor        (Oct 2008)
     111  AW-Pro : Tune AW, -"-                                (Oct 2008)
     112  BW-Pro : Tune BW, -"-                                (Oct 2008)
     113  DW-Pro : Tune DW, -"-                                (Oct 2008)
     114 DWT-Pro : Tune DWT, -"-                               (Oct 2008)
     115  QW-Pro : Tune QW, -"-                                (Oct 2008)
     116 ATLAS-DC2-Pro: ATLAS-DC2 / Rome, -"-                  (Oct 2008)
     117 ACR-Pro : Tune ACR, -"-                               (Oct 2008)
     118  D6-Pro : Tune D6, -"-                                (Oct 2008)
     119 D6T-Pro : Tune D6T, -"-                               (Oct 2008)
    --- Professor's Q2-ordered Perugia Tune : 129 -----------------------
     129 Pro-Q20 : Professor Q2-ordered tune                   (Feb 2009)
    ===== Intermediate and Hybrid Models ================================
     211 APT-Pro : Tune APT, with LEP tune from Professor      (Oct 2008)
     221 Perugia APT  : "Perugia" update of APT-Pro            (Feb 2009)
     226 Perugia APT6 : "Perugia" update of APT-Pro w. CTEQ6L1 (Feb 2009)
    ===== New UE, interleaved pT-ordered showers, annealing CR ==========
    --- Professor Tunes : 310+ (= 300+ with Professor's tune to LEP)
     310   S0-Pro : S0 with updated LEP pars from Professor    (Oct 2008)
     311   S1-Pro : S1 -"-                                     (Oct 2008)
     312   S2-Pro : S2 -"-                                     (Oct 2008)
     313  S0A-Pro : S0A -"-                                    (Oct 2008)
     314 NOCR-Pro : NOCR -"-                                   (Oct 2008)
     315  Old-Pro : Old -"-                                    (Oct 2008)
    --- Peter's Perugia Tunes : 320+ ------------------------------------
     320 Perugia 0 : "Perugia" update of S0-Pro                (Feb 2009)
     321 Perugia HARD : More ISR, More FSR, Less MPI, Less BR, Less HAD
     322 Perugia SOFT : Less ISR, Less FSR, More MPI, More BR, More HAD
     323 Perugia 3 : Alternative to Perugia 0, with different ISR/MPI
                     balance & different scaling to LHC & RHIC (Feb 2009)
     324 Perugia NOCR : "Perugia" update of NOCR-Pro           (Feb 2009)
     325 Perugia * : "Perugia" Tune w. (external) MRSTLO* PDFs (Feb 2009)
     326 Perugia 6 : "Perugia" Tune w. (external) CTEQ6L1 PDFs (Feb 2009)
    --- Professor's pT-ordered Perugia Tune : 329 -----------------------
     329 Pro-pT0   : Professor pT-ordered tune w. S0 CR model  (Feb 2009)
    =====================================================================
    Note: The Perugia tunes all include data from multiple collider
    energies (basically the Tevatron span: 630 - 1960 GeV). In
    particular, the default energy scaling, PARP(90)=0.16, appears to
    be too slow to get simultaneous agreement over the 630 - 1960
    range, regardless of other parameter choices. This was also noted
    by Rick Field and is the reason behind the non-default value of
    PARP(90)=0.25 used in his nominal tunes (not the "T" variants). Of
    the older pT-ordered tunes, only tune S0A, which uses the Tune A
    energy scaling, can therefore be said to have an acceptable
    scaling. Note that the new Perugia tunes still exhibit large
    differences in PARP(90) among themselves, and thus the uncertainty
    is still significant, but at least the new tunes have all been
    tested to give reasonable agreement from 630 GeV to 1960 GeV.
    Note: As before, these tunes can either be invoked by a call to
    PYTUNE with the tune number as argument (must be done prior to
    calling PYINIT) *or* by setting MSTP(5) = tune number. When
    calling PYTUNE directly, make sure to set MSTP(5)=0.
    Note: Since the tunes do not check whether untuned defaults are
    left unaltered by previous calls or user modifications, these
    tunes should in general not be called in sequence, but only once,
    at the start of each run. 
  - PYSLHA: protected some internal PYTHIA particles from read-in of 
    SLHA mass and DECAY tables, in order to avoid incompatibilities with 
    Pythia's hadronization and hadron decay machinery. The SLHA interface is
    intended for read-in of non-SM physics, so we do not consider it a
    strong restriction to protect Pythia's special internal codes and
    all SM particles with masses less than 20 GeV from being
    overwritten by SLHA read-in. People who do wish to alter, say,
    the decay tables of B mesons, should not use the SLHA tables to
    accomplish this, but rather use Pythia's internal updating routine
    PYUPDA.  A modification of the b-quark mass, for example, would
    need to be handled through the PMAS array.
  - Change in PYPTIS for MSTP(67)=1, affecting coherence options for
    the hardest emission in the pT-ordered shower. Previously, only
    values of PARP(67)>1 were allowed. Now also values PARP(67)<1 
    are possible. 
  - Corrected warning message in PYPTMI. The pT-ordered UE model
    requires heavy quarks to be produced at least 5% above threshold,
    in order for there to be some phase space available for the
    subsequent ISR evolution. When this condition failed, it was
    previously reported as the heavy quarks being produced *below*
    threshold. The warning has now been changed to say "too close to
    threshold" instead, and will only be given if two successive
    attempts both trigger this rejection criterion, since it is a
    normal part of ordinary running that some fraction of the produced
    heavy quarks will fail this criterion. 
  - First tries to generate UE with LO* pdfs led to infinite loops in
    PYPTMI. A check has now been added forcing the SEA and VAL
    components to be treated as separately positive definite in the
    code. Also in PYPDFU, a check has been included, forcing the
    derived valence distribution to be positive definite. This was
    observed to be violated at very small Q2 and x even for the
    default CTEQ5L set, so some small changes in current tunes of the
    new interleaved UE framework may result. 
  - New option in PYFSCR to suppress the amount of color annealing in  
    fast-moving string pieces. The amount of suppression is controlled 
    by PARP(77). The default value is PARP(77)=0D0, which corresponds
    to no suppression, i.e., the same behaviour as in previous
    versions. If PARP(77) > 0, then a suppression factor 
                       1/(1+PARP(77)**2*P2AVG)
    is applied to suppress the colour reconnection probability for a
    given string piece, where P2AVG is a measure of the average
    squared momentum that hadrons produced by the string piece would
    have. For hadron colliders, P2AVG is computed using only the
    transverse components of the momenta of the string piece
    endpoints (multiplied by 3/2), while for lepton colliders, also
    the longitudinal component is used. The total momentum of the
    string piece is then divided by the logarithm of the mass of the
    string piece (cut off at 1) to give P2AVG, an estimate of the
    average momentum of the hadrons it produces. Thus, the
    reconnection probability for string pieces that produce
    fast-moving hadrons will be suppressed compared to string pieces
    producing slower-moving hadrons. The motivation for introducing
    such a suppression is, on the theory side, that fast-moving string
    pieces should be less likely to have time to reconnect, and on the
    experimental side, that the pT spectra of charged particles
    measured by CDF appear to become too hard if no such suppression
    is applied. 
  - New option for PYPTFS, can now be called with MODE=3, in which
    case it will use (or create) MCT colour information to locate
    colour partners for radiating partons. PYCTTR updated with 
    possibility to trace a single colour partner rather than entire 
    history.  
  - Bug corrected for the special case when pT-ordered final state
    showers are used in conjunction with the old UE framework. Affects
    Tune APT. Previously, the partons emitted during the Q2-ordered
    ISR phase were not showered correctly by PYPTFS. PYSSPA has now
    been augmented to save the outgoing partons from ISR correctly for
    passing to PYPTFS, and a dedicated call to PYPTFS has been
    included in PYEVNT. Affects Tune APT. A related small bug in
    PYPTFS has been corrected; when a dipole-recoiler of the "right"
    (anti-) colour cannot not be found, the "wrong" partner is now
    used instead. Before, it could sometimes happen that this partner
    was taken to be zero, leading to possible segmentation faults. 
  - Bug corrected in PYEVOL for MSTP(70)=0 and MSTP(70)=1. Affects 
    tune S1, but none of the other tunes in PYTUNE. These options 
    force the ISR evolution to cut off sharply, at a scale defined by
    PARP(62) (for MSTP(70)=0) or at a scale determined by PARP(81) 
    scaled to the current collider energy (for MSTP(70)=1). 
    Previously, invoking these options would also cause the MPI
    evolution to cut off at that scale. This has now been fixed, so 
    that the MPI evolution proceeds independently of the ISR cutoff. 
  - UED: some switches and parameters in IUED and RUED have been
    moved, to make the enumeration more consistent, and to make the 
    separation between the cases with and without gravity-mediated
    decays more clear. In addition, some new possibilities and
    parameters have been added, such as the possibility to give
    Lambda*R instead of Lambda itself, and to switch off radiative
    corrections to the KK masses (on by default). Apologies to current
    users for the lack of backwards compatibility in this update. The
    UED switches and parameters are now (see above for defaults):
     IUED(1) UED ON(=1)/OFF(=0) switch 
     IUED(2) ON(=1)/OFF(=0) switch for gravity mediated decays
     IUED(3) NFLAVOURS Number of KK excitation quark flavours
     IUED(4) N the number of non-universal large extra dimensions 
     IUED(5) Selects whether the code takes Lambda (=0)
             or Lambda*R (=1) as input.
     IUED(6) Radiative corrections to the masses ON (=1) or OFF (=0)
     RUED(1) RINV (1/R) the curvature of the extra dimension
     RUED(2) XMD the (4+N)-dimensional Planck scale
     RUED(3) LAMUED (Lambda cutoff scale), used for IUED(5)=0
     RUED(4) LAMUED/RINV (order of 10-20), used for IUED(5)=1
     PMAS(25,1) Higgs mass.
  - UED: two new functions added: PYGRAW and PYWDKK are now used by
    PYGRAM to perform a numerical integration of the the KK photon
    decay width  to gravitons (when gravity-mediated decays are
    switched on), correcting a numerical problem with the analytical 
    form for IUED(4)=2. The analytical form is still used for
    IUED(4)=4,6. 
  - UED: corrected the d*_Sbar name in the CHAF array in PYDATA. 
  - UED: bug corrected. Some decay channels for KK Z bosons involving
    KK iso-singlets should have been off, but had been left with
    non-zero widths. This is now corrected. Also added 9 new decay
    channels for the KK Z boson.
  - Bug corrected in the mass spectrum of GVMD states in the
    description of photon beams. A square root operation had been
    forgotten, meaning that what should have been the mass was
    actually the squared mass. The intended allowed range
    1 GeV < m < 2*pTmin, where pTmin approx 2 GeV is the multiple
    interactions cutoff scale, thus became 1 GeV < m < 4*pTmin2.
    Also the related kT2 scales in VINT(283) and VINT(284) have
    been the square of what they should have been. Thanks to Kai
    Gallmeister for pointing to this problem.
  - In 6.4.12, the possibility was introduced to add showers off the
    UE partons from MPI in the old UE model, via the switch
    MSTP(152)=1 (default =0). (Note that showers off the MPI are on by
    default in the new interleaved model, and are unaffected by this
    switch.) Switching on such showers in conjunction with pT-ordered
    final-state radiation, MSTJ(41)=11 or MSTJ(41)=12, produced
    somewhat strange results, the origin of which are not yet fully
    understood. Until the interface between the old UE model and
    pT-ordered final-state showers can be checked in detail, the
    showers off MPI in the old model will now default back to the
    Q2-ordered algorithm, regardless of MSTJ(41). This appears to
    produce more consistent results. To be followed up on. May affect
    tunes using MSTJ(41)=11,12 if MSTP(152)=1 is switched on, but no
    previous tunes we are aware of were using that particular combination. 
  - Corrected bug in PYADSH. Changed which final state showering
    routine is called when MINT(35)=2. It should be PYSHOW. 
  - Protected PYFSCR against possible bug when using optimization (only
    observed on Macs using the flags -O2 -fbounds-check). No change in
    physics output should result.

6.4.21 : 13 July 2009
  - Bug corrected in PYRAND, for setting the starting scale for
    multiple parton interactions (underlying event) in external
    processes. For internal processes, PYRAND calls PYSIGH which sets
    all relevant scales. This includes the MPI starting scale VINT(62)
    which is determined by the nature of the process and by the value
    of MSTP(86) (see manual). For external events (e.g., LHEF, HEPEVT,
    etc.), however, PYSIGH is not called, which resulted in VINT(62)
    not being properly initialized, in turn yielding a faulty
    underlying-event evolution. This has now been addressed by making
    sure VINT(62) is explicitly set by PYRAND whenever PYSIGH is not
    called. Since there should normally not be any danger of double
    counting between multiple parton interactions and external
    processes, the default behaviour is to allow MPI evolution over
    all the kinematically allowed phase space, i.e., potentially
    allowing for a small tail of UE jets that could be even harder
    than the scale of the external process. This should not be
    confused with QCD jets due to initial state radiation (ISR) which
    are still always restricted to be below the hard scale for
    external events, and it is only the ISR jets which should be taken
    into account in matching prescriptions - MPI jets should be
    ignored in that context. The only exception to allowing MPI
    evolution over all of phase space is when the external process is
    itself of the same kind as the MPI, i.e., QCD jets. In this case,
    and in this case only, the starting scale should be limited by the
    scale of the hard process in order to avoid double counting. The
    problem is that Pythia has no fail-safe way of knowing whether a
    given external process is of the QCD jets type or not, and hence
    the default, MSTP(86)=2, has been chosen to be to allow MPI
    evolution over all of phase space, with no attempt to determine
    whether the external process is QCD jets or not. When inputting
    external QCD jets events, the non-default option MSTP(86)=1 must
    therefore be used, which forces the multiple parton interactions
    starting scale to be that of the hard processes. Note that top
    pairs are not considered QCD jets in this context, since tops are
    not produced by the multiple parton interactions, hence for top
    production the default MSTP(86)=2 can be retained with no danger
    of double counting. Thanks to L. Mijovic for reporting and helping
    to fix this problem.
  - Change in PYDATA to update the default value of PARP(90) from the
    Pythia 6.2 default of 0.16 to the Tune A value of 0.25. This change 
    does not modify Pythia's predictions for the Tevatron at 1800 GeV, 
    but will change the default energy scaling away from that energy. 
    Specifically, the new default predicts a lower activity in minimum-
    bias events and a smaller underlying event at the LHC. Conversely,
    the new default gives a larger activity at lower CM energies. The
    decision to change this default was made following discussions at
    the energy-scaling workshop at Fermilab, Apr 27-29 2009. 
  - Bug corrected in PYTUNE for tune 329 (Pro-pT0). The value of
    PARP(71) was 4D0, should have been 2D0. The change will slightly 
    reduce the amount of hard FSR activity in umatched non-s-channel 
    processes with this tune. Thanks to A. Buckley for finding this.  
  - Hardcoded a few more default settings in PYTUNE to increase its
    robustness against being called several times in sequence. This is
    still strongly discouraged, however. The only advised way of using
    PYTUNE remains to call it ONCE for each run.  
  - Bug corrected in PYSLHA for automated read-in of QNUMBERS, MASS, 
    and DECAY tables from the header part of LHEF files. The routine
    now skips everything not explicitly inside the <slha>
    tag. Previously, the routine would read everything down to the
    </slha> end tag, leading to potential conflicts with unrelated
    information above the <slha> tag. Thanks to A. Belyaev for
    pointing to this bug.  
  - Bug fix in PYDATA for the decay channels of Majorana neutrinos
    where a charge conjugation was missing for some channels,
    resulting in the same decay mode appearing twice.
  - Bug fix in PYSCAT to handle the case when the incoming parton for
    an external event has particle code >40 or <-40, of particular
    interest for simulating exclusive hadron-hadron processes
  - Bug fix in PYEVNW. Previously, an event vetoed by PYVETO was
    classified as an error. Since Pythia normally stops automatically
    after a small number of errors have been reported (default is 10),
    this led to unintended 'crashes' when using PYVETO with the new
    pT-ordered showers, e.g., in the context of matching with AlpGen. 
  - Protected PYWIDT against possible out-of-bounds addressing caused
    by dummy decay modes inserted by PYSLHA when the branching
    fractions of a read-in DECAY table do not add up to unity.

6.4.22 : 11 November 2009
  - Bug fixes in PYPTIS for massive quark evolution close to threshold
    in annihilation-type processes, for the special case where both
    incoming massive quarks get so close to threshold that creation
    has to be forced. Previously, the algorithm would then always pick
    the Q on side 1 to start with, thus possibly creating an artificial
    asymmetry. This choice has now been randomized to alternate
    between sides 1 and 2. Also, when performing the creation for the
    first Q, the algorithm did not check explicitly whether enough
    phase space was still left for creation of the second Q. This is
    now ensured as well. Thanks to H. Hoeth and S. Kama for pointing
    to issues that led to the identification of this bug. 
  - Change in PYINPR for MSEL = 39, SUSY production. The manual states 
    that MSEL = 39 switches on all SUSY production processes except Higgs
    production. However, previously MSEL = 39 would switch on the SUSY Higgs
    pair production processes 297-301. The meaning of MSEL = 39 has now been
    changed to correspond better to what is written in the manual, i.e., 
    pair production of states with R-parity = -1, leaving out processes 
    297-301. Thanks to M. Johansen for pointing this out. 
  - Protected PYPTFS from compiler-dependent behavior regarding the
    logical evaluation of an IF statement. Noticed by S. Kama. 
  - Enforced initialization of NJN in PYPTIS and of NCHN in
    PYPTMI. Added SAVE statement for NCHN in PYPTMI. Should prevent
    problems experienced with some compilers, notably
    gfortran. Noticed by S. Kama and E. Ozcan.
  - Bug fix in PYEVNW to address problems with how particles point
    back to their ancestors in the pT-ordered shower when
    MSTP(125)<=1. In the new shower, each time a parton acts as a
    "recoiler", it is saved as a new copy with modified
    momentum. Previously, when using the options MSTP(125)<=1, Pythia
    would attempt to compress this history down to a single parton,
    but this gave rise to some inconsistent mother-daughter pointers
    which in turn resulted in some problems in interpreting the event
    history reported by users of the new framework. The "fix" is to
    give up on saving space and instead retain each of the recoiler
    copies separately, without compressing them down to a single
    parton. For MSTP(125)=2 there should be no changes, since this
    option is equivalent to keeping the entire branching history
    without compression.  
  - Updated PYLOGO, Skands now at CERN.

6.4.23 : 09 Jun 2010
  - Bug fix in PYSIGH for interactions after the first in the new 
    interleaved multiple-interactions framework. Previously, the 
    scattering cross sections for additional interactions did not,
    in fact, take into account the PDF shape modifications caused by 
    companion quarks and reduction of the valence content caused
    by previous interactions. There were thus no correlations, e.g., 
    between the signs of two strange quarks when two such were kicked
    out, and the probability to kick out a u quark did not decrease
    if a u quark had already been kicked out. After the bug fix, such
    correlations now appear as expected. Note that this will also 
    lead to changes in the underlying event for all tunes of the
    new framework. In all cases we looked at in combination with the 
    validation runs accompanying this bug fix, the result was < 10%
    increase in the average charged multiplicity. For instance, after
    the fix, the following changes were observed for the Perugia 0
    tune, which is representative of the pT-ordered models:
                               pp @ 7TeV | ppbar @ 2TeV | pp @ 0.2TeV 
      <Nch> for 200-GeV dijets      + 6%           + 9%             -
      <Nch> for 30-GeV dijets       + 4%           + 9%          +10%
      <Nch> for Drell-Yan           + 6%           +11%             - 
      <Nch> for ttbar               + 2%           +10%             -  
      <Nch> for min-bias            + 2%           + 1%          + 2%
    Thus, since the min-bias spectra do not change appreciably, and
    since these were the primary tuning constraints, the tunings
    should still be valid, but will give slightly different
    predictions for other processes after the bug fix. 
  - Added new ATLAS tunes in PYTUNE (see list at beginning of these
    notes) 
    ITUNE   NAME        
      316 : ATLAS MC08 (2008 ATLAS tune of pT-ordered shower. Uses 
            CTEQ6L1 PDFs. Warning: uses Peterson fragmentation
	    function for heavy quarks.)
      330 : ATLAS MC09 (2009 ATLAS tune of pT-ordered shower. Uses LO*
            PDFs.)
      331 : ATLAS MC09c (2009 ATLAS tune of pT-ordered shower. Uses LO*
            PDFs and has retuned CR with respect to original MC09.)
      340 : AMBT1 (2010 ATLAS tune of pT-ordered shower. Uses LO*
            PDFs and agrees better with 7-TeV data.)
    Note: none of these ATLAS tunes have yet been retuned to optimize 
    agreement with LEP data. Their fragmentation properties may
    therefore still not be optimal. 
  - Added new Professor tunes  in PYTUNE (see list at beginning of
    these notes) 
      335 : Pro-PT* (variant of Pro-PTO using MRST LO* PDFs)
      336 : Pro-PT6 (variant of Pro-PTO using CTEQ6L1 PDFs)
      339 : Pro-PT** (variant of PRo-PTO using MRST LO** PDFs)
    Warning: both these and Pro-PTO (329) use MSTP(72)=0, which
    probably restricts the phase space for FSR off ISR too much in
    comparison to both experimental and theoretical studies.
  - Added new Perugia 2010 and Perugia K tunes in PYTUNE (see
    hep-ph/1005.3457 and the list at beginning of these notes)
      327 : Perugia 2010 (2010 update of Perugia 0, with more FSR off
            ISR, more strangeness production, more baryon transport, 
            and modified high-z fragmentation functions.)
      328 : Perugia K (new Perugia variation with K-factor on UE)
    Updated reference to Perugia tunes to new journal writeup.    
  - Added printout in PYTUNE of more relevant parameters, also ones
    that were just left at their default values, in order to
    facilitate comparisons. 
  - Hardcoded defaults in PYTUNE for MSTP(3), MSTP(51), MSTP(52), and 
    MSTP(33) to make tune selections more robust against being called
    in sequence. This is still not recommended and may lead to badly
    initialized tunes. 
  - Bug fixed in PYRVCH for R-parity violating chargino decay widths 
    via the LLE couplings. Previously, each decay of charginos to
    three charged leptons appeared twice with the same decay products 
    but different orderings, thus double counting those modes. This
    double counting has now been removed. Thanks to N.-E. Bomark for
    finding (and correcting) this bug. 
  - Fixed bug in PYSLHA warning statement issued when W, Z, or t
    masses deviate substantially from their physical values. The
    previous 'free' format risked generating crashes.
  - Bug fix in PYPREP to prevent crashes due to free-format write 
    into a character string. Thanks to M. Betancourt for pointing to
    this problem. 
  - Bug fix in PYFSCR. Previously, the algorithm only looked among
    the reconnected partons when deciding where to insert a leftover
    gluon at the end of the algorithm. It now looks among all color 
    lines. This also helps to make the algorithm more stable,
    preventing rare crashes that occurred when no viable insertion
    could be found among the reconnected partons. 
  - Bug fix in PYFSCR. Previously, the Seattle (and the new Paquis)
    type CR models (MSTP(95)=6,7,8,9) would only start from free 
    triplet color tags (either quarks or gluons which only had one
    "side" free). This caused problems with rare events where the
    entire reconnected system had been a gluon loop in the original 
    unconnected topology. The algorithms have now been modified to
    start from any gluon in such cases. 
  - Introduced new color-reconnection (CR) algorithm in PYFSCR,
    obtained with MSTP(95)=8 and =9. The full listing of options for
    MSTP(95) is now:
      MSTP(95) = 0 : No final-state color reconnections.
      MSTP(95) = 1 : Old cut-and-paste style CR.
      MSTP(95) = 2 : Annealing 1, applied to hadron-hadron only
      MSTP(95) = 3 : Annealing 1, applied to all beams
      MSTP(95) = 4 : Annealing 2, applied to hadron-hadron only
      MSTP(95) = 5 : Annealing 2, applied to all beams
      MSTP(95) = 6 : "S" Annealing, applied to hadron-hadron only
      MSTP(95) = 7 : "S" Annealing, applied to all beams
      MSTP(95) = 8 : "P" Annealing, applied to hadron-hadron only
      MSTP(95) = 9 : "P" Annealing, applied to all beams
    In the new "P" (Paquis) model, the reconnection probability for each
    string piece is computed as follows: first, the Thrust axis for
    the event is determined. (For hadron-hadron, this will normally
    coincide with the beam axis.) The number of string pieces in each
    rapidity interval along that axis is then computed, using a
    segmentation of 100 bins in rapidity. The reconnection strength
    PARP(78) is then interpreted as the reconnection probability per
    unit rapidity per string-string overlap, and hence the naive
    probability to keep the string piece unchanged is 
      Pkeep = (1-PARP(78))**(Nbins*DeltaYperBin*<Nstrings-1>)
    where the "-1" in the exponent comes from the string not being
    able to reconnect with itself. In practice the algorithm is
    actually slightly more refined, and the probability to keep the 
    given string piece is constructed as an explicit product over 
    bins, using the number of strings in each bin, rather than the 
    average as shown above. Also, PARP(77) still gives a possibility 
    to suppress CR among high-pT string pieces, similarly to the 
    earlier algorithms.
  - New options in PYSIGH to make it possible to apply a K-factor to
    the QCD multiple-parton-interaction (MPI) cross sections in the
    underlying-event framework independently of any K-factor used for
    the hard trigger process. The new options are:
      MSTP(33) = 10 : The UE QCD cross sections (ISUB=96) are
                      multiplied by PARP(32). Other cross 
		      sections are unchanged.
      MSTP(33) = 11 : The UE QCD cross sections (ISUB=96) are
                      multiplied by PARP(32). Other cross 
		      sections are multiplied by PARP(31). 
    Note: a new tune has been included in PYTUNE that explores this
    possibility, Tune Perugia K, with tune number 328 (see above).
  - Bug fix in PYPTIS, for MSTP(67)=1 and 2. Previously, the scale for
    suppression of hard ISR radiation above the hard-process scale was 
    calculated as SDIP = (p1 + p2)^2, where p1 and p2 are the two 
    color-dipole ends. This is correct for annihilation-type color
    flows, but to better reflect the acceleration of charge also for
    color flows through the diagram, the expression has been changed 
    to SDIP = abs((p1 - p2)^2), which still reduces to sHat for
    massless incoming particles but becomes |tHat| for color flows
    through the diagram. 
  - Updated check for PYDATA linking in PYLOGO, to reflect change of decade
  - Harmonized version numbers in update notes to 6.4.xx throughout
    update notes.
  - Fix to PYWIDX to select proper masses when KF != KC.   Used only by
    technicolor particles.

6.4.24 : 21 Oct 2010
  - Included three new tunes:
      334 : Perugia 10 NOCR (NOCR variant of Perugia 2010)
      341 : Z1 (retune of AMBT1 by Field. Uses CTEQ5L PDFs)
      342 : Z1-LEP (retune of Z1 by Skands. Uses CTEQ5L PDFs)
  - New possibility to write out SLHA DECAY tables from Pythia, using 
    the PYSLHA routine. The new capability can be used by calling
        CALL PYSLHA(4,KF,IRETRN)
    where the key 4 signifies DECAY table writeout, KF should be 
    a PDG code for the particle for which DECAY table writeout is 
    desired, and IRETRN is a return integer which is zero if the call 
    succeeded and non-zero otherwise. Several calls can be issued in 
    sequence, to write out several decay tables. The file onto which 
    the output is written must already be opened by the user at the 
    time PYSLHA is called, and the unit number, LUN, should be given to
    Pythia by setting IMSS(24)=LUN. Many thanks to N.-E. Bomark for 
    providing this new functionality. 
  - Bug fix in PYSLHA regarding the output of the Higgs parameter 
    alpha.  Previously, it would print out 1, instead of the value.
  - Bug fix for SUSY ~t1~t2* production (ISUB=263) in PYSGSU. Removed 
    an erroneous factor 2 in the cross section. 
  - Bug fix in PYWIDT, for decays of W' to a pair of heavy fermions 
    when gV != gA. A term 6(gV^2-gA^2)*Sqrt(m1*m2) was missing in the
    decay width calculation. Thanks to M. Chizhov, see arXiv:0705.3944.
  - Bug fix in PYTUNE, for the AMBT1 tune. Very minor changes
    expected. PARP(77) changed to 1.016 instead of 1.000. PARP(62)
    changed to 1.025 instead of 1.000. Thanks to J. Katzy. 
  - Bug fix in PYFSCR, for MSTP(95)=8 (affecting the Perugia 2010
    tune). Corrected an array-out-of-bounds problem that could
    potentially lead to crashes and/or erroneous results. Thanks 
    to L. Galtieri and D. Mietlicki for pointing to this problem. 
  - Corrected bug in PYTUNE for Perugia SOFT tune (322). Previously,
    PARP(67) was set to 0.5 for this tune, now corrected to 0.25 as in
    arXiv:1005.3457. Thanks to L. Mijovic for pointing to this. 
  - PYRESD: update.   Color flow for 3-body decays of the form
    3 -> 3 + 3 + 3bar (where 3 is a color triplet) is now handled.
    This could arise in a decay mode defined through an SLHA file.
    The color of the mother flows to the first decay product, and
    the remaining two are paired as a color singlet.  Other flows
    involving four colored particles are not yet handled.
  - Introduced "generic" processes of the kind 2->1->2 and 2->2,
    with process codes 481 and 482, respectively.   These rely on
    reading and interpreting an SLHA file.   The process initial
    and final state is defined through the SLHA decay table of
    a new particle KF=9900001.  This particle should have only
    two decays modes, the first specifying the initial state of
    Standard Model partons and the second the final state, which
    may include particles defined through the SLHA file.  The
    QNUMBERS definition of particle KF=9900001 determines the 
    color flow, and the charge and color must be consistent with
    the two decay modes.   For process 482, the cross section is
    determined by a flat matrix element.   For process 481, a
    Breit-Wigner propagator for particle KF=9900001 is included.
    The reported cross section includes no couplings or
    branching ratio factors, and is determined only by the 
    parton distribution functions and particle masses.  
  - Bug fix to PYSGSU to allow selection of incoming partons from
    the PDF for gluino + ino production.   The default behavior
    of including all relevant incoming partons is unchanged.
  - Modification of PYPTFS to allow for final state showering of
    any, heavy color octet particle in the pT-ordered shower.
  - Modification of PYTBDY to allow the user to turn off matrix
    element weighting in certain SUSY decays when MSTJ(47)=0.
  - Bug fix to PYSCAT to allow simulation of processes 297-301,
    which were inadvertently disabled in version 6.4.17.
  - PYRESD modified to allow 4-body decays of resonances, as long as
    one of the decay particles is inert (doesn't shower).
  - PYPTIS modified PYPTIS to allow the option to turn off QED
    ISR when MSTP(61)=0.

6.4.25 : 23 Mar 2011
  - Included 18 new tunes:
      343 : Z2 (retune of Z1 by Field. Uses CTEQ6L1 PDFs)
      344 : Z2-LEP (retune of Z2 by Skands. Uses CTEQ6L1 PDFs)
      350 : Perugia 2011 (update of Perugia 2010. Uses CTEQ5L PDFs)
      351 : Perugia 2011 radHi (with alphaS(pT/2))
      352 : Perugia 2011 radLo (with alphaS(2*pT))
      353 : Perugia 2011 mpiHi (with larger alphaS for MPI)
      354 : Perugia 2011 noCR (without color reconnections)
      355 : Perugia 2011 M (Uses LO** PDFs)
      356 : Perugia 2011 C (Uses CTEQ6L1 PDFs)
      357 : Perugia 2011 T16 (with PARP(90)=0.16 away from 7 TeV)
      358 : Perugia 2011 T32 (with PARP(90)=0.32 away from 7 TeV)
      359 : Perugia 2011 Tevatron
      360 : S Global (Schulz-Skands Global fit. Uses CTEQ5L PDFs)
      361 : S 7000   (Schulz-Skands optimized at 7 TeV)
      362 : S 1960   (Schulz-Skands optimized at 1.96 TeV)
      363 : S 1800   (Schulz-Skands optimized at 1.8 TeV)
      364 : S 900    (Schulz-Skands optimized at 900 GeV)
      365 : S 630    (Schulz-Skands optimized at 630 GeV)
  - Bug fix in PYPTFS for QED and QCD final-state showers in systems
    with color and/or charge flowing through the process. 
      QED example: W+ -> l+ nu. 
      QCD/QED examples: t -> b W or leptoquark decays. 
    Since dipoles in resoance decays must be formed *within* the
    resonant system (in order to preserve its mass and hence its
    Breit-Wigner shape), any charge or color lines through the 
    process cannot be totally correctly treated (a formal
    initial-final type of dipole connection would be needed). In
    previous versions, this ambiguity could lead to gluons using
    leptons or photons as recoilers far down the shower, leading to
    larger-than-expected momentum kicks being imparted to those
    particles. The shower routine has now been updated so that colored
    particles will always attempt to find colored recoilers,
    preferably with the correct color charge sign. When there are
    ambiguities between several such potential recoilers, the pairing
    with the smallest dot product is made. The treatment of QED
    dipoles has also been updated, so that a QED emitter keeps the
    same recoiler all the way through the shower evolution. Thanks to 
    H. Bachacou, V. Bansal, and several others for helping to chart
    this bug.   
  - PYSLHA:  Fixed typo when reporting W mass. Also changed several
    write FORMAT statements to be compatible with gfortran.
  - New feature in PYPTFS. Added a new QED dipole between the q and
    qbar emerging from a g->qqbar splitting in the shower.
  - Improved option in PYPTFS for switching radiation from a specific 
    particle off via MSTJ(39), see update notes for 6.4.19. The flag 
    is now also active for particles produced during the showering and   
    not just for particles coming into the showering.
  - New behaviour in the pT-ordered initial-state shower, for 
    MSTP(67) = 0, i.e., when coherence is switched off for the first
    emission. (This non-default setting is normally only used in
    matching contexts, hence should not affect the default behaviour
    of the code.) The phase space boundaries for the first parton
    shower emission are now set as follows:                       
    MSTP(67) = 0 : ISR: Step function at PARP(67)*QF**2
                        (this was previously just QF**2) 
                   FSR: Step function at PARP(71)*QF**2
    Note: for external processes, QF = SCALUP. 
    This allows an independent handle to adjust the shower starting
    scale consistently for ISR and FSR which can be useful for testing
    purposes. In cases where the factorization scale is clearly
    interpretable as the pT scale of a QCD jet, we recommend setting
    PARP(67) = PARP(71) = 1D0.
 
6.4.26 : 16 Nov 2011
  - Corrected bug in PYTUNE affecting calls of PYTUNE with a constant
    argument. Previously, the argument was assumed to be non-constant
    and could be modified by PYTUNE. This would result in compilation
    problems and/or crashes if the argument being passes was
    constant. This behaviour has now been corrected, so the argument
    is never modified and instead a local copy of it is made inside
    PYTUNE. Thanks to M. Wobisch for pointing to this bug.
  - Fixed bug in PYEVOL causing potential inconsistencies for hard 
    processes with 3 or more outgoing partons. Previously, if the
    evolution failed so that PYEVOL had to restore the original hard
    system and start over, only the two first partons of the hard
    process would be restored correctly. This has now been corrected
    so that hard systems of arbitrary size are treated correctly. 
  - PYSGSU:  Fixed sign error in gluino contribution to squark-antisquark
    production, process 274, to agree with Dawson, Eichten, and Quigg.
  - PYSLHA:  Added flexibility to control decay modes from an SLHA file,
    like internal decay modes.   By setting the MDME value of a particular
    decay mode to a negative value, the effect of the positive value is
    obtained.   For example, MDME(5001,1)=-4 will result in the setting
    of MDME(5001,1)=4, where 5001 is a decay mode inserted from an SLHA
    file.  
  - PYUPIN:  Output cross section information to LHE file when using
    internal processes.
  - PYRESD:  Bug fix that allows gluino decay modes to be controlled
    with MDME options. 
    
6.4.27 : 12 Dec 2012
  - Included 10 new Perugia 2012 tunes, based on Perugia 2011 but with
    the central tune based on CTEQ6L1 and some new variations. The
    following PYTUNE/MSTP(5) numbers have been added:
       370 P12       : Retune of Perugia 2011 w CTEQ6L1          (Oct 2012)
       371 P12-radHi : Variation with alphaS(pT/2) 
       372 P12-radLo : Variation with alphaS(2pT)
       373 P12-mpiHi : Variation with more semi-hard MPI -> more UE
       374 P12-loCR  : Variation using lower CR strength -> more Nch
       375 P12-noCR  : Variation without any color reconnections
       376 P12-FL    : Variation with more longitudinal fragmentation
       377 P12-FT    : Variation with more transverse fragmentation
       378 P12-M8LO  : Variation using MSTW 2008 LO PDFs     
       379 P12-LO**  : Variation using MRST LO** PDFs     
    In particular, the "loCR" variation is intended to give a less
    extreme option for variation than the "noCR" one. Note that we
    also include MSTW 2008 LO and LO** variants, and that tunes 376
    and 377 represent variations of the string fragmentation a, b, 
    and pT parameters. Tune 376 has a more "longitudinal"
    fragmentation, with lower non-perturbative pT, and a harder long-
    itudinal f(z), whereas tune 377 has the opposite. As in Perugia
    2011, all shower alphaS parameters use the same LambaQCD values.
  - Included 5 new ATLAS UE/MB tunes, from the A*T2B family, with
    PYTUNE/MSTP(5) numbers
        345 AMBT2B-CT6L : 2nd ATLAS MB tune, vers 'B', w CTEQ6L1 (Jul 2011)
        346 AUET2B-CT6L : UE tune accompanying AMBT2B            (Jul 2011)
        347 AUET2B-CT66 : AUET2 with CTEQ 6.6 NLO PDFs           (Nov 2011)
	348 AUET2B-CT10 : AUET2 with CTEQ 10 NLO PDFs            (Nov 2011)
	349 AUET2B-NN21 : AUET2 with NNPDF 2.1 NLO PDFs          (Nov 2011)
    Note that tunes 347, 348, and 349 use NLO PDFs. Tunes using NLO
    PDFs are not recommended for physics studies, hence these are
    mostly for illustration purposes. 
  - Updated PYFSCR to include two additional colour-reconnection models from 
    the Uppsala group, the SCI and GAL models, which were previously only 
    available as private modifications. They have now been included in the 
    standard Pythia distrubution and are accessible via four new options 
    in MSTP(95): CR Type
               = 11 : Soft Color Interaction (SCI model)
               = 12 : SCI without diffraction (no inter-remnant CR)
               = 13 : Generalized Area Law (GAL model)
               = 14 : GAL without diffraction (no inter-remnant CR)
    The existing options 2-9 (for colour annealing, used eg by the Perugia 
    tunes) remain unmodified. The new options are documented in 
       SCI: hep-ph/9508386, by Edin, Ingelman, and Rathsman
       GAL: hep-ph/9812423, by Rathsman
    Thanks to Johan Rathsman for providing this implementation. 
    Also included new highly preliminary tunes in PYTUNE,
    with the following PYTUNE/MSTP(5) tune numbers:
        1201   SCI 0 : Soft-Colour-Interaction model Org pars (Dec 1998)
	1202   SCI 1 : SCI 0. Tevatron MB retuned (Skands)    (Oct 2006)
        1401   GAL 0 : Generalized area-law model. Org pars   (Dec 1998)
        1402   GAL 1 : GAL 0. Tevatron MB retuned (Skands)    (Oct 2006)
  - Bug fix in PYWIDT for decay modes using tabulated branching
    fractions. Previously, modes with zero partial width could 
    contribute with a NaN via undefined secondary widths, producing
    erroneous results with some compilers. The code has now been
    protected by explicitly zeroing the partial width in such cases. 
  - Bug fix in PYPTIS, for suppressed pT-ordered "power
    showers". Previously, the scale at which the suppression would
    kick in was only computed correctly for color-annihilation
    channels, for t-channel exchanges, and for channels without
    intermediate s-channel resonances. For more complicated topologies
    in which the initial-state color flows to a final-state parton
    through intermediate resonances, however, the suppression scale
    could become extremely small, effectively killing ISR in such
    topologies. This has now been addressed by letting the IF dipole
    be defined by the incoming parton and the last final-state parton
    to carry the matching color tag. If no such matching parton is
    found, the default is to use the other incoming parton to define
    the suppression scale.
  - Bug fix in PYPTMI and PYPTIS to prevent heavy companion quarks
    (b and c quarks) from being picked up by MPI and ISR evolution. 
    This is necessary since perturbative intertwining (joining of two
    chains) was never implemented in practice and heavy quarks have to
    disappear from the remnant object before the perturbative cutoff
    is reached. The consequence of the fix is that MPI are still
    sensitive to heavy-quark companion PDFs, but such quarks are  
    reclassified as sea quarks when assigning them. For ISR, the PDF
    weight for backwards evolution to a heavy companion quark is
    forced to zero. Thanks to O. Brandt for pointing to this issue. 
  - Bug fix in PYSLHA for DECAY tables including decays to
    gravitinos. Previously, the read-in of the gravitino mass into
    RMSS(21) and the setting to allow gravitino decays, IMSS(11) > 0, 
    were not handled automatically by PYSLHA, resulting in errors when
    attempting to read SLHA files for scenarios in which the LSP can 
    decay to X + gravitino. If a decay mode including a gravitino is
    found, PYSLHA now sets IMSS(11) = 2, to indicate that decays to
    gravitinos should be allowed, and the value of the mass (in eV) is
    stored in RMSS(21). Thanks to M. Magro for pointing to this issue.
  - Bug fix in PYSTRF to update MSTU(90) correctly when string
    junctions are present in the event record and/or when infinite
    loops are encountered (MSTU(90) is used for bookkeeping of the 
    fragmentation of c and b hadrons). Thanks to M. Jaffre for
    pointing to this problem.  

6.4.28 : 05 Sep 2013
  - Bug fix in the handling of final-state radiation in decays into a
    gluon, specifically q* -> q g, where an incorrect handling of the
    matrix element matching leads to a strong suppression of radiation
    from the gluon. 
  - Bug fix in PYUPEV to handle certain processes, such as VV->VV
    scattering (MSUB=71-77), where a mother index would become negative.
  - Included four new Perugia 2012 variations, one of which is
    motivated by theory uncertainties on min-bias, two others having
    been requested by the LHC experiments, and the last exploring a
    different set of fragmentation parameters, obtained by the
    Innsbruck group. Also included 9 new complete ee+pp tunes provided 
    by G. Rudolph and N. Firdous (Innsbruck), 3 of which use LO PDFs,
    3 using NLO ones, and 3 LO* ones. Note that we do not recommend
    using NLO or LO* PDFs for serious soft-physics studies, but include 
    them nonetheless to illustrate the results that one obtains with them. 
    MSTP(5) = 
      380 P12-MB2     : same as Perugia 2012, with PARP(87)=0D0    (2013)
      381 P12-ueHi    : Variation with higher UE (lower pT0)       (2013)
      382 P12-ueLo    : Variation with lower UE (higher pT0)       (2013)
      383 P12-IBK     : Perugia 2012 with IBK ee fragmentation parameters
      390 IBK-CTEQ5L  : Innsbruck ee+pp tune with CTEQ5 LO PDFs    (2013)
      391 IBK-CTEQ6LL :       with CTEQ6LL LO PDFs
      392 IBK-MSTW08LO:       with MSTW08 LO PDFS
      393 IBK-CTEQ66NLO:      with CTEQ66 NLO PDFs
      394 IBK-CT10NLO :       with CT10 NLO PDFs
      395 IBK-MSTW08NLO:      with MSTW08 NLO PDFs
      396 IBK-MSTW08LO*:      with MSTW07 LO* PDFs
      397 IBK-MRSTLO**:       with MRSTMCal (LO**) PDFs
      398 IBK-CT09MC2 :       with CTEQ09MC2 PDFs  
    The variation with PARP(87)=0 (see manual) has a slightly improved 
    behaviour at very low minimum-bias multiplicities. The new P12 UE 
    variations are complementary to the existing mpiHi variation (373). 
    Where the latter increases the amount of semi-hard MPI scatterings 
    (i.e., the amount of MPI minijets) by increasing the alphaS associated 
    with MPI, the new variations adjust the soft pT0 scale and hence affect 
    the amount of soft MPI produced, without changing the rate of hard MPI. 
    The energy-scaling of the new variations has also been chosen
    conservatively, so that the activity of the "ueLo" variation scales 
    slower with CM energy (i.e., pT0 scales up faster) than for "ueHi". 
  - Extended PYSLHA to be able to handle more than 25 new particles
    introduced via QNUMBERS blocks in SLHA files and/or LHEF
    headers. By recycling previously unused space in the particle
    arrays, the maximum number of QNUMBERS blocks that can be stored
    is now at 130 new particles, at which point the code will advise
    you to move to PYTHIA 8 instead. Thanks to A. Belyaev for
    pointing to this problem and for help testing the updated PYSHLA.  
  - Modification of PYPTFS for the case of interleaved showers, where
    final state showers occur after initial state showers. In
    the previous version, a soft parton from ISR could form a dipole
    with a final state parton, leading to a suppression of hard final-
    state radiation. In the corrected version, a final-final dipole is 
    selected, using other hard partons in the final state, unless
    the pT-scale is at or below the scale of the original initial-final
    dipole. Thanks to Gavin Salam for noticing a problem in jet
    masses for certain kinematic configurations. See also the study
    reported in section 8.4 of Dasgupta et al., arXiv:1307.0007. 
    Note that this correction slightly increases the average amount of
    FSR in all tunes of the pT-ordered shower. Thus, for instance, the 
    average multiplicity of minimum-bias events at LHC (7 TeV) increases 
    by 1 - 2 per cent in this version relative to previous PYTHIA 6 versions. 
  - Identified a bug in final states including exactly one vector boson
    and one quark. This could be a native Pythia process or an
    external LHE event. In this case, a matrix element correction is
    applied to FSR assuming the decay q -> q V.   The effect appears
    to be minimal, and no solution is implemented at this time.