HERWIG/src/sasbeh.f

   1
   2 C
   3
   4 C*********************************************************************
   5
   6 C
   7
   8       SUBROUTINE SASBEH(KF,X,Q2,P2,PM2,XPBH)
   9
  10 C...Purpose: to evaluate the Bethe-Heitler cross section for
  11
  12 C...heavy flavour production.
  13
  14       DATA AEM2PI/0.0011614/
  15
  16 C
  17
  18 C...Reset output.
  19
  20       XPBH=0.
  21
  22       SIGBH=0.
  23
  24 C
  25
  26 C...Check kinematics limits.
  27
  28       IF(X.GE.Q2/(4.*PM2+Q2+P2)) RETURN
  29
  30       W2=Q2*(1.-X)/X-P2
  31
  32       BETA2=1.-4.*PM2/W2
  33
  34       IF(BETA2.LT.1E-10) RETURN
  35
  36       BETA=SQRT(BETA2)
  37
  38       RMQ=4.*PM2/Q2
  39
  40 C
  41
  42 C...Simple case: P2 = 0.
  43
  44       IF(P2.LT.1E-4) THEN
  45
  46         IF(BETA.LT.0.99) THEN
  47
  48           XBL=LOG((1.+BETA)/(1.-BETA))
  49
  50         ELSE
  51
  52           XBL=LOG((1.+BETA)**2*W2/(4.*PM2))
  53
  54         ENDIF
  55
  56         SIGBH=BETA*(8.*X*(1.-X)-1.-RMQ*X*(1.-X))+
  57
  58      &  XBL*(X**2+(1.-X)**2+RMQ*X*(1.-3.*X)-0.5*RMQ**2*X**2)
  59
  60 C
  61
  62 C...Complicated case: P2 > 0, based on approximation of
  63
  64 C...C.T. Hill and G.G. Ross, Nucl. Phys. B148 (1979) 373
  65
  66       ELSE
  67
  68         RPQ=1.-4.*X**2*P2/Q2
  69
  70         IF(RPQ.GT.1E-10) THEN
  71
  72           RPBE=SQRT(RPQ*BETA2)
  73
  74           IF(RPBE.LT.0.99) THEN
  75
  76             XBL=LOG((1.+RPBE)/(1.-RPBE))
  77
  78             XBI=2.*RPBE/(1.-RPBE**2)
  79
  80           ELSE
  81
  82             RPBESN=4.*PM2/W2+(4.*X**2*P2/Q2)*BETA2
  83
  84             XBL=LOG((1.+RPBE)**2/RPBESN)
  85
  86             XBI=2.*RPBE/RPBESN
  87
  88           ENDIF
  89
  90           SIGBH=BETA*(6.*X*(1.-X)-1.)+
  91
  92      &    XBL*(X**2+(1.-X)**2+RMQ*X*(1.-3.*X)-0.5*RMQ**2*X**2)+
  93
  94      &    XBI*(2.*X/Q2)*(PM2*X*(2.-RMQ)-P2*X)
  95
  96         ENDIF
  97
  98       ENDIF
  99
 100 C
 101
 102 C...Multiply by charge-squared etc. to get parton distribution.
 103
 104       CHSQ=1./9.
 105
 106       IF(IABS(KF).EQ.2.OR.IABS(KF).EQ.4) CHSQ=4./9.
 107
 108       XPBH=3.*CHSQ*AEM2PI*X*SIGBH
 109
 110 C
 111
 112       RETURN
 113
 114       END