HERWIG/src/hwualf.f

   1
   2 CDECK  ID>, HWUALF.
   3
   4 *CMZ :-        -15/07/92  14.08.45  by  Mike Seymour
   5
   6 *-- Author :    Bryan Webber
   7
   8 C-----------------------------------------------------------------------
   9
  10       FUNCTION HWUALF(IOPT,SCALE)
  11
  12 C-----------------------------------------------------------------------
  13
  14 C     STRONG COUPLING CONSTANT
  15
  16 C     IOPT.EQ.0  INITIALIZES
  17
  18 C         .EQ.1  TWO-LOOP, FLAVOUR THRESHOLDS
  19
  20 C         .EQ.2  RATIO OF ABOVE TO ONE-LOOP
  21
  22 C                WITH 5-FLAVOUR BETA, LAMBDA=QCDL3
  23
  24 C         .EQ.3  ONE-LOOP WITH 5-FLAVOUR BETA, LAMBDA=QCDL3
  25
  26 C-----------------------------------------------------------------------
  27
  28       INCLUDE 'HERWIG61.INC'
  29
  30       DOUBLE PRECISION HWUALF,SCALE,KAFAC,B3,B4,B5,B6,C3,C4,C5,C6,C35,
  31
  32      & C45,C65,D35,RHO,RAT,RLF,DRH,EPS
  33
  34       INTEGER IOPT,ITN
  35
  36       SAVE B3,B4,B5,B6,C3,C4,C5,C6,C35,C45,C65,D35
  37
  38       DATA EPS/1.D-6/
  39
  40       IF (IOPT.EQ.0) THEN
  41
  42 C---INITIALIZE CONSTANTS
  43
  44         CAFAC=FLOAT(NCOLO)
  45
  46         CFFAC=FLOAT(NCOLO**2-1)/(2.*CAFAC)
  47
  48         B3=((11.*CAFAC)- 6.)/(12.*PIFAC)
  49
  50         B4=((11.*CAFAC)- 8.)/(12.*PIFAC)
  51
  52         B5=((11.*CAFAC)-10.)/(12.*PIFAC)
  53
  54         B6=((11.*CAFAC)-12.)/(12.*PIFAC)
  55
  56         BETAF=6.*PIFAC*B5
  57
  58         C3=((17.*CAFAC**2)-(5.*CAFAC+3.*CFFAC)*3.)/(24.*PIFAC**2)/B3**2
  59
  60         C4=((17.*CAFAC**2)-(5.*CAFAC+3.*CFFAC)*4.)/(24.*PIFAC**2)/B4**2
  61
  62         C5=((17.*CAFAC**2)-(5.*CAFAC+3.*CFFAC)*5.)/(24.*PIFAC**2)/B5**2
  63
  64         C6=((17.*CAFAC**2)-(5.*CAFAC+3.*CFFAC)*6.)/(24.*PIFAC**2)/B6**2
  65
  66         KAFAC=CAFAC*(67./18.-PIFAC**2/6.)-25./9.
  67
  68 C---QCDLAM IS 5-FLAVOUR LAMBDA-MS-BAR AT LARGE X OR Z
  69
  70 C---QCDL5  IS 5-FLAVOUR LAMBDA-MC
  71
  72         QCDL5=QCDLAM*EXP(KAFAC/(4.*PIFAC*B5))/SQRT(2.D0)
  73
  74 C---COMPUTE THRESHOLD MATCHING
  75
  76         RHO=2.*LOG(RMASS(6)/QCDL5)
  77
  78         RAT=LOG(RHO)/RHO
  79
  80         C65=(B5/(1.-C5*RAT)-B6/(1.-C6*RAT))*RHO
  81
  82         RHO=2.*LOG(RMASS(5)/QCDL5)
  83
  84         RAT=LOG(RHO)/RHO
  85
  86         C45=(B5/(1.-C5*RAT)-B4/(1.-C4*RAT))*RHO
  87
  88         RHO=2.*LOG(RMASS(4)/QCDL5)
  89
  90         RAT=LOG(RHO)/RHO
  91
  92         C35=(B4/(1.-C4*RAT)-B3/(1.-C3*RAT))*RHO+C45
  93
  94 C---FIND QCDL3
  95
  96         D35=-1./(B3*C35)
  97
  98         DO 10 ITN=1,100
  99
 100           RAT=LOG(D35)/D35
 101
 102           RLF=B3*D35/(1.-C3*RAT)
 103
 104           DRH=B3*(RLF+C35)*D35**2/((1.-2.*C3*RAT+C3/D35)*RLF**2)
 105
 106           D35=D35-DRH
 107
 108           IF (ABS(DRH).LT.EPS*D35) GOTO 20
 109
 110    10   CONTINUE
 111
 112    20   QCDL3=QCDL5*EXP(0.5*D35)
 113
 114       ENDIF
 115
 116       IF (SCALE.LE.QCDL5) CALL HWWARN('HWUALF',51,*999)
 117
 118       RHO=2.*LOG(SCALE/QCDL5)
 119
 120       IF (IOPT.EQ.3) THEN
 121
 122         IF (RHO.LE.D35) CALL HWWARN('HWUALF',52,*999)
 123
 124         HWUALF=1./(B5*(RHO-D35))
 125
 126         RETURN
 127
 128       ENDIF
 129
 130       RAT=LOG(RHO)/RHO
 131
 132       IF (SCALE.GT.RMASS(6)) THEN
 133
 134         RLF=B6*RHO/(1.-C6*RAT)+C65
 135
 136       ELSEIF (SCALE.GT.RMASS(5)) THEN
 137
 138         RLF=B5*RHO/(1.-C5*RAT)
 139
 140       ELSEIF (SCALE.GT.RMASS(4)) THEN
 141
 142         RLF=B4*RHO/(1.-C4*RAT)+C45
 143
 144       ELSE
 145
 146         RLF=B3*RHO/(1.-C3*RAT)+C35
 147
 148       ENDIF
 149
 150       IF (RLF.LE.ZERO) CALL HWWARN('HWUALF',53,*999)
 151
 152       IF (IOPT.EQ.1) THEN
 153
 154         HWUALF=1./RLF
 155
 156       ELSE
 157
 158         HWUALF=B5*(RHO-D35)/RLF
 159
 160         IF (HWUALF.GT.ONE) CALL HWWARN('HWUALF',54,*999)
 161
 162       ENDIF
 163
 164       RETURN
 165
 166  999  HWUALF=ZERO
 167
 168       END