hijing/src/ulalps.f

0001
0002 C*********************************************************************
0003
0004       FUNCTION ULALPS(Q2)
0005
0006 C...Purpose: to give the value of alpha_strong.
0007       COMMON/LUDAT1/MSTU(200),PARU(200),MSTJ(200),PARJ(200)
0008       SAVE /LUDAT1/
0009       COMMON/LUDAT2/KCHG(500,3),PMAS(500,4),PARF(2000),VCKM(4,4)
0010       SAVE /LUDAT2/
0011
0012 C...Constant alpha_strong trivial.
0013       IF(MSTU(111).LE.0) THEN
0014         ULALPS=PARU(111)
0015         MSTU(118)=MSTU(112)
0016         PARU(117)=0.
0017         PARU(118)=PARU(111)
0018         RETURN
0019       ENDIF
0020
0021 C...Find effective Q2, number of flavours and Lambda.
0022       Q2EFF=Q2
0023       IF(MSTU(115).GE.2) Q2EFF=MAX(Q2,PARU(114))
0024       NF=MSTU(112)
0025       ALAM2=PARU(112)**2
0026   100 IF(NF.GT.MAX(2,MSTU(113))) THEN
0027         Q2THR=PARU(113)*PMAS(NF,1)**2
0028         IF(Q2EFF.LT.Q2THR) THEN
0029           NF=NF-1
0030           ALAM2=ALAM2*(Q2THR/ALAM2)**(2./(33.-2.*NF))
0031           GOTO 100
0032         ENDIF
0033       ENDIF
0034   110 IF(NF.LT.MIN(8,MSTU(114))) THEN
0035         Q2THR=PARU(113)*PMAS(NF+1,1)**2
0036         IF(Q2EFF.GT.Q2THR) THEN
0037           NF=NF+1
0038           ALAM2=ALAM2*(ALAM2/Q2THR)**(2./(33.-2.*NF))
0039           GOTO 110
0040         ENDIF
0041       ENDIF
0042       IF(MSTU(115).EQ.1) Q2EFF=Q2EFF+ALAM2
0043       PARU(117)=SQRT(ALAM2)
0044
0045 C...Evaluate first or second order alpha_strong.
0046       B0=(33.-2.*NF)/6.
0047       ALGQ=LOG(Q2EFF/ALAM2)
0048       IF(MSTU(111).EQ.1) THEN
0049         ULALPS=PARU(2)/(B0*ALGQ)
0050       ELSE
0051         B1=(153.-19.*NF)/6.
0052         ULALPS=PARU(2)/(B0*ALGQ)*(1.-B1*LOG(ALGQ)/(B0**2*ALGQ))
0053       ENDIF
0054       MSTU(118)=NF
0055       PARU(118)=ULALPS
0056
0057       RETURN
0058       END