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2 | C********************************************************************* | |
3 | ||
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. | |
9 | ||
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), | |
17 | &XPDIR(-6:6) | |
18 | SAVE /PYINT8/ | |
19 | ||
20 | C...Temporary array. | |
21 | DIMENSION XPGA(-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. | |
27 | DATA ALAM/0.20/ | |
28 | C...Mixture u/(u+d), = 0.5 for incoherent and = 0.8 for coherent sum. | |
29 | DATA FRACU/0.8/ | |
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/ | |
34 | ||
35 | C...Reset output. | |
36 | F2GM=0. | |
37 | DO 100 KFL=-6,6 | |
38 | XPDFGM(KFL)=0. | |
39 | XPVMD(KFL)=0. | |
40 | XPANL(KFL)=0. | |
41 | XPANH(KFL)=0. | |
42 | XPBEH(KFL)=0. | |
43 | XPDIR(KFL)=0. | |
44 | 100 CONTINUE | |
45 | ||
46 | C...Set k0 cut-off parameter as function of set used. | |
47 | IF(ISET.LE.2) THEN | |
48 | AK0=0.6 | |
49 | ELSE | |
50 | AK0=2. | |
51 | ENDIF | |
52 | ||
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. | |
55 | P2MX=MAX(P2,AK0**2) | |
56 | CALL PYGVMD(ISET,1,X,Q2,P2MX,ALAM,XPGA) | |
57 | XFVAL=XPGA(1)-XPGA(2) | |
58 | XPGA(1)=XPGA(2) | |
59 | XPGA(-1)=XPGA(-2) | |
60 | FACUD=AEM*(1./FRHO+1./FOMEGA)*(PMRHO**2/(PMRHO**2+P2))**2 | |
61 | FACS=AEM*(1./FPHI)*(PMPHI**2/(PMPHI**2+P2))**2 | |
62 | DO 110 KFL=-5,5 | |
63 | XPVMD(KFL)=(FACUD+FACS)*XPGA(KFL) | |
64 | 110 CONTINUE | |
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 | |
71 | ||
72 | C...Call anomalous parametrization for d + u + s. | |
73 | CALL PYGANO(-3,X,Q2,P2MX,ALAM,XPGA) | |
74 | DO 120 KFL=-5,5 | |
75 | XPANL(KFL)=XPGA(KFL) | |
76 | 120 CONTINUE | |
77 | ||
78 | C...Call anomalous parametrization for c and b. | |
79 | CALL PYGANO(4,X,Q2,P2MX,ALAM,XPGA) | |
80 | DO 130 KFL=-5,5 | |
81 | XPANH(KFL)=XPGA(KFL) | |
82 | 130 CONTINUE | |
83 | CALL PYGANO(5,X,Q2,P2MX,ALAM,XPGA) | |
84 | DO 140 KFL=-5,5 | |
85 | XPANH(KFL)=XPANH(KFL)+XPGA(KFL) | |
86 | 140 CONTINUE | |
87 | ||
88 | C...Call Bethe-Heitler term expression for charm and bottom. | |
89 | CALL PYGBEH(4,X,Q2,P2,PMC**2,XPBH) | |
90 | XPBEH(4)=XPBH | |
91 | XPBEH(-4)=XPBH | |
92 | CALL PYGBEH(5,X,Q2,P2,PMB**2,XPBH) | |
93 | XPBEH(5)=XPBH | |
94 | XPBEH(-5)=XPBH | |
95 | ||
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) | |
99 | DO 150 KFL=-5,5 | |
100 | XPDIR(KFL)=XPGA(KFL) | |
101 | 150 CONTINUE | |
102 | ENDIF | |
103 | ||
104 | C...Store result in output array. | |
105 | DO 160 KFL=-5,5 | |
106 | CHSQ=1./9. | |
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) | |
111 | 160 CONTINUE | |
112 | ||
113 | RETURN | |
114 | END |