PYTHIA/jetset/luxkfl.F

   1
   2 C*********************************************************************
   3
   4       SUBROUTINE LUXKFL(KFL,ECM,ECMC,KFLC)
   5
   6 C...Purpose: to select flavour for produced qqbar pair.
   7       COMMON/LUDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200)
   8       COMMON/LUDAT2/KCHG(500,3),PMAS(500,4),PARF(2000),VCKM(4,4)
   9       SAVE /LUDAT1/,/LUDAT2/
  10
  11 C...Calculate maximum weight in QED or QFD case.
  12       IF(MSTJ(102).LE.1) THEN
  13         RFMAX=4./9.
  14       ELSE
  15         POLL=1.-PARJ(131)*PARJ(132)
  16         SFF=1./(16.*PARU(102)*(1.-PARU(102)))
  17         SFW=ECMC**4/((ECMC**2-PARJ(123)**2)**2+(PARJ(123)*PARJ(124))**2)
  18         SFI=SFW*(1.-(PARJ(123)/ECMC)**2)
  19         VE=4.*PARU(102)-1.
  20         HF1I=SFI*SFF*(VE*POLL+PARJ(132)-PARJ(131))
  21         HF1W=SFW*SFF**2*((VE**2+1.)*POLL+2.*VE*(PARJ(132)-PARJ(131)))
  22         RFMAX=MAX(4./9.*POLL-4./3.*(1.-8.*PARU(102)/3.)*HF1I+
  23      &  ((1.-8.*PARU(102)/3.)**2+1.)*HF1W,1./9.*POLL+2./3.*
  24      &  (-1.+4.*PARU(102)/3.)*HF1I+((-1.+4.*PARU(102)/3.)**2+1.)*HF1W)
  25       ENDIF
  26
  27 C...Choose flavour. Gives charge and velocity.
  28       NTRY=0
  29   100 NTRY=NTRY+1
  30       IF(NTRY.GT.100) THEN
  31         CALL LUERRM(14,'(LUXKFL:) caught in an infinite loop')
  32         KFLC=0
  33         RETURN
  34       ENDIF
  35       KFLC=KFL
  36       IF(KFL.LE.0) KFLC=1+INT(MSTJ(104)*RLU(0))
  37       MSTJ(93)=1
  38       PMQ=ULMASS(KFLC)
  39       IF(ECM.LT.2.*PMQ+PARJ(127)) GOTO 100
  40       QF=KCHG(KFLC,1)/3.
  41       VQ=1.
  42       IF(MOD(MSTJ(103),2).EQ.1) VQ=SQRT(MAX(0.,1.-(2.*PMQ/ECMC)**2))
  43
  44 C...Calculate weight in QED or QFD case.
  45       IF(MSTJ(102).LE.1) THEN
  46         RF=QF**2
  47         RFV=0.5*VQ*(3.-VQ**2)*QF**2
  48       ELSE
  49         VF=SIGN(1.,QF)-4.*QF*PARU(102)
  50         RF=QF**2*POLL-2.*QF*VF*HF1I+(VF**2+1.)*HF1W
  51         RFV=0.5*VQ*(3.-VQ**2)*(QF**2*POLL-2.*QF*VF*HF1I+VF**2*HF1W)+
  52      &  VQ**3*HF1W
  53         IF(RFV.GT.0.) PARJ(171)=MIN(1.,VQ**3*HF1W/RFV)
  54       ENDIF
  55
  56 C...Weighting or new event (radiative photon). Cross-section update.
  57       IF(KFL.LE.0.AND.RF.LT.RLU(0)*RFMAX) GOTO 100
  58       PARJ(158)=PARJ(158)+1.
  59       IF(ECMC.LT.2.*PMQ+PARJ(127).OR.RFV.LT.RLU(0)*RF) KFLC=0
  60       IF(MSTJ(107).LE.0.AND.KFLC.EQ.0) GOTO 100
  61       IF(KFLC.NE.0) PARJ(159)=PARJ(159)+1.
  62       PARJ(144)=PARJ(157)*PARJ(159)/PARJ(158)
  63       PARJ(148)=PARJ(144)*86.8/ECM**2
  64
  65       RETURN
  66       END