HERWIG/src/hwdhgf.f

   1
   2 CDECK  ID>, HWDHGF.
   3
   4 *CMZ :-        -02/05/91  11.11.45  by  Federico Carminati
   5
   6 *-- Author :    Mike Seymour
   7
   8 C-----------------------------------------------------------------------
   9
  10       FUNCTION HWDHGF(X,Y)
  11
  12 C-----------------------------------------------------------------------
  13
  14 C  CALCULATE THE DOUBLE BREIT-WIGNER INTEGRAL
  15
  16 C  X=(EMV/EMH)**2 , Y=EMV*GAMV/EMH**2
  17
  18 C-----------------------------------------------------------------------
  19
  20       INCLUDE 'HERWIG61.INC'
  21
  22       DOUBLE PRECISION HWDHGF,X,Y,CHANGE,X1,X2,FAC1,FAC2,TH1,TH2,TH1HI,
  23
  24      & TH1LO,TH2HI,TH2LO,X2MAX,SQFAC
  25
  26       INTEGER NBIN,IBIN1,IBIN2
  27
  28 C  CHANGE IS THE POINT WHERE DIRECT INTEGRATION BEGINS TO CONVERGE
  29
  30 C  FASTER THAN STANDARD BREIT-WIGNER SUBSTITUTION
  31
  32       DATA CHANGE,NBIN/0.425,25/
  33
  34       HWDHGF=0
  35
  36       IF (Y.LT.ZERO) RETURN
  37
  38       IF (X.GT.CHANGE) THEN
  39
  40 C---DIRECT INTEGRATION
  41
  42         FAC1=0.25 / NBIN
  43
  44         DO 200 IBIN1=1,NBIN
  45
  46           X1=(IBIN1-0.5) * FAC1
  47
  48           FAC2=( (1-SQRT(X1))**2-X1 ) / NBIN
  49
  50           DO 100 IBIN2=1,NBIN
  51
  52             X2=(IBIN2-0.5) * FAC2 + X1
  53
  54             SQFAC=1+X1**2+X2**2-2*(X1+X2+X1*X2)
  55
  56             IF (SQFAC.LT.ZERO) GOTO 100
  57
  58             HWDHGF=HWDHGF + 2.
  59
  60      &        * ((1-X1-X2)**2+8*X1*X2)
  61
  62      &        * SQRT(SQFAC)
  63
  64      &        / ((X1-X)**2+Y**2) *Y
  65
  66      &        / ((X2-X)**2+Y**2) *Y
  67
  68      &        * FAC1*FAC2
  69
  70  100      CONTINUE
  71
  72  200    CONTINUE
  73
  74       ELSE
  75
  76 C---INTEGRATION USING TAN THETA SUBSTITUTIONS
  77
  78         TH1LO=ATAN((0-X)/Y)
  79
  80         TH1HI=ATAN((1-X)/Y)
  81
  82         FAC1=(TH1HI-TH1LO) / NBIN
  83
  84         DO 400 IBIN1=1,NBIN
  85
  86           TH1=(IBIN1-0.5) * FAC1 + TH1LO
  87
  88           X1=Y*TAN(TH1) + X
  89
  90           X2MAX=MIN(X1,(1-SQRT(X1))**2)
  91
  92           TH2LO=ATAN((0-X)/Y)
  93
  94           TH2HI=ATAN((X2MAX-X)/Y)
  95
  96           FAC2=(TH2HI-TH2LO) / NBIN
  97
  98           DO 300 IBIN2=1,NBIN
  99
 100             TH2=(IBIN2-0.5) * FAC2 + TH2LO
 101
 102             X2=Y*TAN(TH2) + X
 103
 104             SQFAC=1+X1**2+X2**2-2*(X1+X2+X1*X2)
 105
 106             IF (SQFAC.LT.ZERO) GOTO 300
 107
 108             HWDHGF=HWDHGF + 2.
 109
 110      &        * ((1-X1-X2)**2+8*X1*X2)
 111
 112      &        * SQRT(SQFAC)
 113
 114      &        * FAC1 * FAC2
 115
 116  300      CONTINUE
 117
 118  400    CONTINUE
 119
 120       ENDIF
 121
 122       HWDHGF=HWDHGF/(PIFAC*PIFAC)
 123
 124       END