C********************************************************************* C...PYX3JT C...Selects the kinematical variables of three-jet events. SUBROUTINE PYX3JT(NJET,CUT,KFL,ECM,X1,X2) C...Double precision and integer declarations. IMPLICIT DOUBLE PRECISION(A-H, O-Z) IMPLICIT INTEGER(I-N) INTEGER PYK,PYCHGE,PYCOMP C...Commonblocks. COMMON/PYDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200) SAVE /PYDAT1/ C...Local array. DIMENSION ZHUP(5,12) SAVE C...Coefficients of Zhu second order parametrization. DATA ((ZHUP(IC1,IC2),IC2=1,12),IC1=1,5)/ &18.29D0, 89.56D0, 4.541D0, -52.09D0, -109.8D0, 24.90D0, &11.63D0, 3.683D0, 17.50D0,0.002440D0, -1.362D0,-0.3537D0, &11.42D0, 6.299D0, -22.55D0, -8.915D0, 59.25D0, -5.855D0, &-32.85D0, -1.054D0, -16.90D0,0.006489D0,-0.8156D0,0.01095D0, &7.847D0, -3.964D0, -35.83D0, 1.178D0, 29.39D0, 0.2806D0, &47.82D0, -12.36D0, -56.72D0, 0.04054D0,-0.4365D0, 0.6062D0, &5.441D0, -56.89D0, -50.27D0, 15.13D0, 114.3D0, -18.19D0, &97.05D0, -1.890D0, -139.9D0, 0.08153D0,-0.4984D0, 0.9439D0, &-17.65D0, 51.44D0, -58.32D0, 70.95D0, -255.7D0, -78.99D0, &476.9D0, 29.65D0, -239.3D0, 0.4745D0, -1.174D0, 6.081D0/ C...Dilogarithm of x for x<0.5 (x>0.5 obtained by analytic trick). DILOG(X)=X+X**2/4D0+X**3/9D0+X**4/16D0+X**5/25D0+X**6/36D0+ &X**7/49D0 C...Event type. Mass effect factors and other common constants. MSTJ(120)=2 MSTJ(121)=0 PMQ=PYMASS(KFL) QME=(2D0*PMQ/ECM)**2 IF(MSTJ(109).NE.1) THEN CUTL=LOG(CUT) CUTD=LOG(1D0/CUT-2D0) IF(MSTJ(109).EQ.0) THEN CF=4D0/3D0 CN=3D0 TR=2D0 WTMX=MIN(20D0,37D0-6D0*CUTD) IF(MSTJ(110).EQ.2) WTMX=2D0*(7.5D0+80D0*CUT) ELSE CF=1D0 CN=0D0 TR=12D0 WTMX=0D0 ENDIF C...Alpha_strong and effects of optimized Q^2 scale. Maximum weight. ALS2PI=PARU(118)/PARU(2) WTOPT=0D0 IF(MSTJ(111).EQ.1) WTOPT=(33D0-2D0*MSTU(112))/6D0* & LOG(PARJ(169))*ALS2PI WTMAX=MAX(0D0,1D0+WTOPT+ALS2PI*WTMX) C...Choose three-jet events in allowed region. 100 NJET=3 110 Y13L=CUTL+CUTD*PYR(0) Y23L=CUTL+CUTD*PYR(0) Y13=EXP(Y13L) Y23=EXP(Y23L) Y12=1D0-Y13-Y23 IF(Y12.LE.CUT) GOTO 110 IF(Y13**2+Y23**2+2D0*Y12.LE.2D0*PYR(0)) GOTO 110 C...Second order corrections. IF(MSTJ(101).EQ.2.AND.MSTJ(110).LE.1) THEN Y12L=LOG(Y12) Y13M=LOG(1D0-Y13) Y23M=LOG(1D0-Y23) Y12M=LOG(1D0-Y12) IF(Y13.LE.0.5D0) Y13I=DILOG(Y13) IF(Y13.GE.0.5D0) Y13I=1.644934D0-Y13L*Y13M-DILOG(1D0-Y13) IF(Y23.LE.0.5D0) Y23I=DILOG(Y23) IF(Y23.GE.0.5D0) Y23I=1.644934D0-Y23L*Y23M-DILOG(1D0-Y23) IF(Y12.LE.0.5D0) Y12I=DILOG(Y12) IF(Y12.GE.0.5D0) Y12I=1.644934D0-Y12L*Y12M-DILOG(1D0-Y12) WT1=(Y13**2+Y23**2+2D0*Y12)/(Y13*Y23) WT2=CF*(-2D0*(CUTL-Y12L)**2-3D0*CUTL-1D0+3.289868D0+ & 2D0*(2D0*CUTL-Y12L)*CUT/Y12)+ & CN*((CUTL-Y12L)**2-(CUTL-Y13L)**2-(CUTL-Y23L)**2- & 11D0*CUTL/6D0+67D0/18D0+1.644934D0-(2D0*CUTL-Y12L)*CUT/Y12+ & (2D0*CUTL-Y13L)*CUT/Y13+(2D0*CUTL-Y23L)*CUT/Y23)+ & TR*(2D0*CUTL/3D0-10D0/9D0)+ & CF*(Y12/(Y12+Y13)+Y12/(Y12+Y23)+(Y12+Y23)/Y13+(Y12+Y13)/Y23+ & Y13L*(4D0*Y12**2+2D0*Y12*Y13+4D0*Y12*Y23+Y13*Y23)/ & (Y12+Y23)**2+Y23L*(4D0*Y12**2+2D0*Y12*Y23+4D0*Y12*Y13+ & Y13*Y23)/(Y12+Y13)**2)/WT1+ & CN*(Y13L*Y13/(Y12+Y23)+Y23L*Y23/(Y12+Y13))/WT1+(CN-2D0*CF)* & ((Y12**2+(Y12+Y13)**2)*(Y12L*Y23L-Y12L*Y12M-Y23L* & Y23M+1.644934D0-Y12I-Y23I)/(Y13*Y23)+(Y12**2+(Y12+Y23)**2)* & (Y12L*Y13L-Y12L*Y12M-Y13L*Y13M+1.644934D0-Y12I-Y13I)/ & (Y13*Y23)+(Y13**2+Y23**2)/(Y13*Y23*(Y13+Y23))- & 2D0*Y12L*Y12**2/(Y13+Y23)**2-4D0*Y12L*Y12/(Y13+Y23))/WT1- & CN*(Y13L*Y23L-Y13L*Y13M-Y23L*Y23M+1.644934D0-Y13I-Y23I) IF(1D0+WTOPT+ALS2PI*WT2.LE.0D0) MSTJ(121)=1 IF(1D0+WTOPT+ALS2PI*WT2.LE.WTMAX*PYR(0)) GOTO 110 PARJ(156)=(WTOPT+ALS2PI*WT2)/(1D0+WTOPT+ALS2PI*WT2) ELSEIF(MSTJ(101).EQ.2.AND.MSTJ(110).EQ.2) THEN C...Second order corrections; Zhu parametrization of ERT. ZX=(Y23-Y13)**2 ZY=1D0-Y12 IZA=0 DO 120 IY=1,5 IF(ABS(CUT-0.01D0*IY).LT.0.0001D0) IZA=IY 120 CONTINUE IF(IZA.NE.0) THEN IZ=IZA WT2=ZHUP(IZ,1)+ZHUP(IZ,2)*ZX+ZHUP(IZ,3)*ZX**2+(ZHUP(IZ,4)+ & ZHUP(IZ,5)*ZX)*ZY+(ZHUP(IZ,6)+ZHUP(IZ,7)*ZX)*ZY**2+ & (ZHUP(IZ,8)+ZHUP(IZ,9)*ZX)*ZY**3+ZHUP(IZ,10)/(ZX-ZY**2)+ & ZHUP(IZ,11)/(1D0-ZY)+ZHUP(IZ,12)/ZY ELSE IZ=100D0*CUT WTL=ZHUP(IZ,1)+ZHUP(IZ,2)*ZX+ZHUP(IZ,3)*ZX**2+(ZHUP(IZ,4)+ & ZHUP(IZ,5)*ZX)*ZY+(ZHUP(IZ,6)+ZHUP(IZ,7)*ZX)*ZY**2+ & (ZHUP(IZ,8)+ZHUP(IZ,9)*ZX)*ZY**3+ZHUP(IZ,10)/(ZX-ZY**2)+ & ZHUP(IZ,11)/(1D0-ZY)+ZHUP(IZ,12)/ZY IZ=IZ+1 WTU=ZHUP(IZ,1)+ZHUP(IZ,2)*ZX+ZHUP(IZ,3)*ZX**2+(ZHUP(IZ,4)+ & ZHUP(IZ,5)*ZX)*ZY+(ZHUP(IZ,6)+ZHUP(IZ,7)*ZX)*ZY**2+ & (ZHUP(IZ,8)+ZHUP(IZ,9)*ZX)*ZY**3+ZHUP(IZ,10)/(ZX-ZY**2)+ & ZHUP(IZ,11)/(1D0-ZY)+ZHUP(IZ,12)/ZY WT2=WTL+(WTU-WTL)*(100D0*CUT+1D0-IZ) ENDIF IF(1D0+WTOPT+2D0*ALS2PI*WT2.LE.0D0) MSTJ(121)=1 IF(1D0+WTOPT+2D0*ALS2PI*WT2.LE.WTMAX*PYR(0)) GOTO 110 PARJ(156)=(WTOPT+2D0*ALS2PI*WT2)/(1D0+WTOPT+2D0*ALS2PI*WT2) ENDIF C...Impose mass cuts (gives two jets). For fixed jet number new try. X1=1D0-Y23 X2=1D0-Y13 X3=1D0-Y12 IF(4D0*Y23*Y13*Y12/X3**2.LE.QME) NJET=2 IF(MOD(MSTJ(103),4).GE.2.AND.IABS(MSTJ(101)).LE.1.AND.QME*X3+ & 0.5D0*QME**2+(0.5D0*QME+0.25D0*QME**2)*((1D0-X2)/(1D0-X1)+ & (1D0-X1)/(1D0-X2)).GT.(X1**2+X2**2)*PYR(0)) NJET=2 IF(MSTJ(101).EQ.-1.AND.NJET.EQ.2) GOTO 100 C...Scalar gluon model (first order only, no mass effects). ELSE 130 NJET=3 140 X3=SQRT(4D0*CUT**2+PYR(0)*((1D0-CUT)**2-4D0*CUT**2)) IF(LOG((X3-CUT)/CUT).LE.PYR(0)*LOG((1D0-2D0*CUT)/CUT)) GOTO 140 YD=SIGN(2D0*CUT*((X3-CUT)/CUT)**PYR(0)-X3,PYR(0)-0.5D0) X1=1D0-0.5D0*(X3+YD) X2=1D0-0.5D0*(X3-YD) IF(4D0*(1D0-X1)*(1D0-X2)*(1D0-X3)/X3**2.LE.QME) NJET=2 IF(MSTJ(102).GE.2) THEN IF(X3**2-2D0*(1D0+X3)*(1D0-X1)*(1D0-X2)*PARJ(171).LT. & X3**2*PYR(0)) NJET=2 ENDIF IF(MSTJ(101).EQ.-1.AND.NJET.EQ.2) GOTO 130 ENDIF RETURN END