**************************** * * * 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. 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 at generator-level in min-bias at the Tevatron. Further retuning could be needed to restore agreement with other distributions, such as (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 tag. Between and , however, it risked crashing due to attempting SLHA read-in of non-SLHA information. PYSLHA now exits when either , , or the first 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 at generator-level in min-bias at the Tevatron. The (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 (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 tag. Previously, the routine would read everything down to the end tag, leading to potential conflicts with unrelated information above the 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 for 200-GeV dijets + 6% + 9% - for 30-GeV dijets + 4% + 9% +10% for Drell-Yan + 6% +11% - for ttbar + 2% +10% - 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*) 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.