2 C*********************************************************************
4 C...The following routines are adapted from
5 C...SaSgam - parton distributions of the photon
6 C...by Gerhard A. Schuler and Torbjorn Sjostrand
7 C...For further information see CERN-TH/95-62.
8 C...The version found here is NOT suitable for standalone usage.
10 SUBROUTINE PYGGAM(ISET,X,Q2,P2,F2GM,XPDFGM)
11 C...Purpose: to construct the F2 and parton distributions of the photon
12 C...by summing homogeneous (VMD) and inhomogeneous (anomalous) terms.
13 C...For F2, c and b are included by the Bethe-Heitler formula;
14 C...in the 'MSbar' scheme additionally a Cgamma term is added.
15 DIMENSION XPDFGM(-6:6)
16 COMMON/PYINT8/XPVMD(-6:6),XPANL(-6:6),XPANH(-6:6),XPBEH(-6:6),
22 C...Charm and bottom masses (low to compensate for J/psi etc.).
23 DATA PMC/1.3/, PMB/4.6/
24 C...alpha_em and alpha_em/(2*pi).
25 DATA AEM/0.007297/, AEM2PI/0.0011614/
26 C...Lambda value for 4 flavours.
28 C...Mixture u/(u+d), = 0.5 for incoherent and = 0.8 for coherent sum.
30 C...VMD couplings f_V**2/(4*pi).
31 DATA FRHO/2.20/, FOMEGA/23.6/, FPHI/18.4/
32 C...Masses for rho (=omega) and phi.
33 DATA PMRHO/0.770/, PMPHI/1.020/
46 C...Set k0 cut-off parameter as function of set used.
53 C...Call VMD parametrization for d quark and use to give rho+omega+ phi.
54 C...Note scale choice and dipole dampening for off-shell photon.
56 CALL PYGVMD(ISET,1,X,Q2,P2MX,ALAM,XPGA)
60 FACUD=AEM*(1./FRHO+1./FOMEGA)*(PMRHO**2/(PMRHO**2+P2))**2
61 FACS=AEM*(1./FPHI)*(PMPHI**2/(PMPHI**2+P2))**2
63 XPVMD(KFL)=(FACUD+FACS)*XPGA(KFL)
65 XPVMD(1)=XPVMD(1)+(1.-FRACU)*FACUD*XFVAL
66 XPVMD(2)=XPVMD(2)+FRACU*FACUD*XFVAL
67 XPVMD(3)=XPVMD(3)+FACS*XFVAL
68 XPVMD(-1)=XPVMD(-1)+(1.-FRACU)*FACUD*XFVAL
69 XPVMD(-2)=XPVMD(-2)+FRACU*FACUD*XFVAL
70 XPVMD(-3)=XPVMD(-3)+FACS*XFVAL
72 C...Call anomalous parametrization for d + u + s.
73 CALL PYGANO(-3,X,Q2,P2MX,ALAM,XPGA)
78 C...Call anomalous parametrization for c and b.
79 CALL PYGANO(4,X,Q2,P2MX,ALAM,XPGA)
83 CALL PYGANO(5,X,Q2,P2MX,ALAM,XPGA)
85 XPANH(KFL)=XPANH(KFL)+XPGA(KFL)
88 C...Call Bethe-Heitler term expression for charm and bottom.
89 CALL PYGBEH(4,X,Q2,P2,PMC**2,XPBH)
92 CALL PYGBEH(5,X,Q2,P2,PMB**2,XPBH)
96 C...For MSbar subtraction call C^gamma term expression for d, u, s.
97 IF(ISET.EQ.2.OR.ISET.EQ.4) THEN
98 CALL PYGDIR(X,Q2,P2,AK0,XPGA)
104 C...Store result in output array.
107 IF(IABS(KFL).EQ.2.OR.IABS(KFL).EQ.4) CHSQ=4./9.
108 XPF2=XPVMD(KFL)+XPANL(KFL)+XPBEH(KFL)+XPDIR(KFL)
109 IF(KFL.NE.0) F2GM=F2GM+CHSQ*XPF2
110 XPDFGM(KFL)=XPVMD(KFL)+XPANL(KFL)+XPANH(KFL)