HERWIG/src/hwbspn.f

   1
   2 CDECK  ID>, HWBSPN.
   3
   4 *CMZ :-        -26/04/91  11.11.54  by  Bryan Webber
   5
   6 *-- Author :    Ian Knowles
   7
   8 C-----------------------------------------------------------------------
   9
  10       SUBROUTINE HWBSPN
  11
  12 C-----------------------------------------------------------------------
  13
  14 C     Constructs appropriate spin density/decay matrix for parton
  15
  16 C     in hard subprocess, othwise zero. Assignments based upon
  17
  18 C     Comp. Phys. Comm. 58 (1990) 271.
  19
  20 C-----------------------------------------------------------------------
  21
  22       INCLUDE 'HERWIG61.INC'
  23
  24       DOUBLE PRECISION C,V12,V23,V13,TR,C1,C2,C3,R1(2),R2(2)
  25
  26       INTEGER IST
  27
  28       SAVE R1,R2,V12
  29
  30       IF (IERROR.NE.0) RETURN
  31
  32       IST=MOD(ISTHEP(NEVPAR),10)
  33
  34 C Assumed partons processed in the order IST=1,2,3,4
  35
  36       IF (IPROC.GE.100.AND.IPROC.LE.116) THEN
  37
  38 C  An e+e- ---> qqbar g event
  39
  40          IF (IDPAR(2).EQ.13) THEN
  41
  42             RHOPAR(1,2)=GPOLN
  43
  44             RHOPAR(2,2)=0.
  45
  46             RETURN
  47
  48          ENDIF
  49
  50       ELSEIF (IPRO.EQ.15.OR.IPRO.EQ.17) THEN
  51
  52          IF (IHPRO.EQ. 7.OR.IHPRO.EQ. 8.OR.
  53
  54      &       IHPRO.EQ.10.OR.IHPRO.EQ.11.OR.
  55
  56      &       IHPRO.EQ.15.OR.IHPRO.EQ.16.OR.
  57
  58      &      (IHPRO.GE.21.AND.IHPRO.LE.31)) THEN
  59
  60 C A hard 2 --- > 2 QCD subprocess involving gluons
  61
  62             IF (IST.EQ.2) THEN
  63
  64                CALL HWVEQU(2,RHOPAR(1,2),R1(1))
  65
  66                C=GCOEF(2)/GCOEF(1)
  67
  68                DECPAR(1,2)=C*R1(1)
  69
  70                DECPAR(2,2)=C*R1(2)
  71
  72                RETURN
  73
  74             ELSEIF (IST.EQ.3) THEN
  75
  76                CALL HWVEQU(2,RHOPAR(1,2),R2(1))
  77
  78                V12=R1(1)*R2(1)+R1(2)*R2(2)
  79
  80                TR=1./(GCOEF(1)+GCOEF(2)*V12)
  81
  82                RHOPAR(1,2)= (GCOEF(3)*R1(1)+GCOEF(4)*R2(1))*TR
  83
  84                RHOPAR(2,2)=-(GCOEF(3)*R1(2)+GCOEF(4)*R2(2))*TR
  85
  86                RETURN
  87
  88             ELSEIF (IST.EQ.4) THEN
  89
  90                V13=R1(1)*DECPAR(1,2)+R1(2)*DECPAR(2,2)
  91
  92                V23=R2(1)*DECPAR(1,2)+R2(2)*DECPAR(2,2)
  93
  94                TR=1./(GCOEF(1)+GCOEF(2)*V12+GCOEF(3)*V13+GCOEF(4)*V23)
  95
  96                C1=(GCOEF(2)+GCOEF(5))*TR
  97
  98                C2=(GCOEF(3)+GCOEF(6))*TR
  99
 100                C3=(GCOEF(4)+GCOEF(6))*TR
 101
 102                RHOPAR(1,2)=C1*DECPAR(1,2)+C2*R2(1)+C3*R1(1)
 103
 104                RHOPAR(2,2)=C1*DECPAR(2,2)-C2*R1(2)-C3*R2(2)
 105
 106                RETURN
 107
 108             ENDIF
 109
 110          ENDIF
 111
 112       ELSEIF (IPRO.EQ.16) THEN
 113
 114 C A gluon fusion ---> Higgs event
 115
 116          IF (IST.EQ.2) THEN
 117
 118             DECPAR(1,2)=RHOPAR(1,2)
 119
 120             DECPAR(2,2)=-RHOPAR(2,2)
 121
 122             RETURN
 123
 124          ENDIF
 125
 126       ENDIF
 127
 128       CALL HWVZRO(2,RHOPAR(1,2))
 129
 130       CALL HWVZRO(2,DECPAR(1,2))
 131
 132       END