4 *CMZ :- -15/07/92 14.08.45 by Mike Seymour
6 *-- Author : Bryan Webber
8 C-----------------------------------------------------------------------
10 FUNCTION HWUALF(IOPT,SCALE)
12 C-----------------------------------------------------------------------
14 C STRONG COUPLING CONSTANT
16 C IOPT.EQ.0 INITIALIZES
18 C .EQ.1 TWO-LOOP, FLAVOUR THRESHOLDS
20 C .EQ.2 RATIO OF ABOVE TO ONE-LOOP
22 C WITH 5-FLAVOUR BETA, LAMBDA=QCDL3
24 C .EQ.3 ONE-LOOP WITH 5-FLAVOUR BETA, LAMBDA=QCDL3
26 C-----------------------------------------------------------------------
28 INCLUDE 'HERWIG61.INC'
30 DOUBLE PRECISION HWUALF,SCALE,KAFAC,B3,B4,B5,B6,C3,C4,C5,C6,C35,
32 & C45,C65,D35,RHO,RAT,RLF,DRH,EPS
36 SAVE B3,B4,B5,B6,C3,C4,C5,C6,C35,C45,C65,D35
42 C---INITIALIZE CONSTANTS
46 CFFAC=FLOAT(NCOLO**2-1)/(2.*CAFAC)
48 B3=((11.*CAFAC)- 6.)/(12.*PIFAC)
50 B4=((11.*CAFAC)- 8.)/(12.*PIFAC)
52 B5=((11.*CAFAC)-10.)/(12.*PIFAC)
54 B6=((11.*CAFAC)-12.)/(12.*PIFAC)
58 C3=((17.*CAFAC**2)-(5.*CAFAC+3.*CFFAC)*3.)/(24.*PIFAC**2)/B3**2
60 C4=((17.*CAFAC**2)-(5.*CAFAC+3.*CFFAC)*4.)/(24.*PIFAC**2)/B4**2
62 C5=((17.*CAFAC**2)-(5.*CAFAC+3.*CFFAC)*5.)/(24.*PIFAC**2)/B5**2
64 C6=((17.*CAFAC**2)-(5.*CAFAC+3.*CFFAC)*6.)/(24.*PIFAC**2)/B6**2
66 KAFAC=CAFAC*(67./18.-PIFAC**2/6.)-25./9.
68 C---QCDLAM IS 5-FLAVOUR LAMBDA-MS-BAR AT LARGE X OR Z
70 C---QCDL5 IS 5-FLAVOUR LAMBDA-MC
72 QCDL5=QCDLAM*EXP(KAFAC/(4.*PIFAC*B5))/SQRT(2.D0)
74 C---COMPUTE THRESHOLD MATCHING
76 RHO=2.*LOG(RMASS(6)/QCDL5)
80 C65=(B5/(1.-C5*RAT)-B6/(1.-C6*RAT))*RHO
82 RHO=2.*LOG(RMASS(5)/QCDL5)
86 C45=(B5/(1.-C5*RAT)-B4/(1.-C4*RAT))*RHO
88 RHO=2.*LOG(RMASS(4)/QCDL5)
92 C35=(B4/(1.-C4*RAT)-B3/(1.-C3*RAT))*RHO+C45
102 RLF=B3*D35/(1.-C3*RAT)
104 DRH=B3*(RLF+C35)*D35**2/((1.-2.*C3*RAT+C3/D35)*RLF**2)
108 IF (ABS(DRH).LT.EPS*D35) GOTO 20
112 20 QCDL3=QCDL5*EXP(0.5*D35)
116 IF (SCALE.LE.QCDL5) CALL HWWARN('HWUALF',51,*999)
118 RHO=2.*LOG(SCALE/QCDL5)
122 IF (RHO.LE.D35) CALL HWWARN('HWUALF',52,*999)
124 HWUALF=1./(B5*(RHO-D35))
132 IF (SCALE.GT.RMASS(6)) THEN
134 RLF=B6*RHO/(1.-C6*RAT)+C65
136 ELSEIF (SCALE.GT.RMASS(5)) THEN
138 RLF=B5*RHO/(1.-C5*RAT)
140 ELSEIF (SCALE.GT.RMASS(4)) THEN
142 RLF=B4*RHO/(1.-C4*RAT)+C45
146 RLF=B3*RHO/(1.-C3*RAT)+C35
150 IF (RLF.LE.ZERO) CALL HWWARN('HWUALF',53,*999)
158 HWUALF=B5*(RHO-D35)/RLF
160 IF (HWUALF.GT.ONE) CALL HWWARN('HWUALF',54,*999)