PYTHIA/jetset/ulalps.F

   1
   2 C*********************************************************************
   3
   4       FUNCTION ULALPS(Q2)
   5
   6 C...Purpose: to give the value of alpha_strong.
   7       COMMON/LUDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200)
   8       COMMON/LUDAT2/KCHG(500,3),PMAS(500,4),PARF(2000),VCKM(4,4)
   9       SAVE /LUDAT1/,/LUDAT2/
  10
  11 C...Constant alpha_strong trivial.
  12       IF(MSTU(111).LE.0) THEN
  13         ULALPS=PARU(111)
  14         MSTU(118)=MSTU(112)
  15         PARU(117)=0.
  16         PARU(118)=PARU(111)
  17         RETURN
  18       ENDIF
  19
  20 C...Find effective Q2, number of flavours and Lambda.
  21       Q2EFF=Q2
  22       IF(MSTU(115).GE.2) Q2EFF=MAX(Q2,PARU(114))
  23       NF=MSTU(112)
  24       ALAM2=PARU(112)**2
  25   100 IF(NF.GT.MAX(2,MSTU(113))) THEN
  26         Q2THR=PARU(113)*PMAS(NF,1)**2
  27         IF(Q2EFF.LT.Q2THR) THEN
  28           NF=NF-1
  29           ALAM2=ALAM2*(Q2THR/ALAM2)**(2./(33.-2.*NF))
  30           GOTO 100
  31         ENDIF
  32       ENDIF
  33   110 IF(NF.LT.MIN(8,MSTU(114))) THEN
  34         Q2THR=PARU(113)*PMAS(NF+1,1)**2
  35         IF(Q2EFF.GT.Q2THR) THEN
  36           NF=NF+1
  37           ALAM2=ALAM2*(ALAM2/Q2THR)**(2./(33.-2.*NF))
  38           GOTO 110
  39         ENDIF
  40       ENDIF
  41       IF(MSTU(115).EQ.1) Q2EFF=Q2EFF+ALAM2
  42       PARU(117)=SQRT(ALAM2)
  43
  44 C...Evaluate first or second order alpha_strong.
  45       B0=(33.-2.*NF)/6.
  46       ALGQ=LOG(MAX(1.0001,Q2EFF/ALAM2))
  47       IF(MSTU(111).EQ.1) THEN
  48         ULALPS=MIN(PARU(115),PARU(2)/(B0*ALGQ))
  49       ELSE
  50         B1=(153.-19.*NF)/6.
  51         ULALPS=MIN(PARU(115),PARU(2)/(B0*ALGQ)*(1.-B1*LOG(ALGQ)/
  52      &  (B0**2*ALGQ)))
  53       ENDIF
  54       MSTU(118)=NF
  55       PARU(118)=ULALPS
  56
  57       RETURN
  58       END