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21886bb6 | 1 | #include "pdf/pilot.h" |
2 | C...SaSgam - parton distributions of the photon | |
3 | C...by Gerhard A. Schuler and Torbjorn Sjostrand | |
4 | C...For further information see preprint CERN-TH/95-62 and LU TP 95-6: | |
5 | C...Low- and high-mass components of the photon distribution functions | |
6 | C...Program last changed on 21 March 1995. | |
7 | ||
8 | C...The user should only need to call the SASGAM routine, | |
9 | C...which in turn calls the auxiliary routines SASVM1, SASAN1, | |
10 | C...SASBEH and SASDIR. The package is self-contained. | |
11 | ||
12 | C...One particular aspect of these parametrizations is that F2 for | |
13 | C...the photon is not obtained just as the charge-squared-weighted | |
14 | C...sum of quark distributions, but differ in the treatment of | |
15 | C...heavy flavours (in F2 the DIS relation W2 = Q2*(1-x)/x restricts | |
16 | C...the kinematics range of heavy-flavour production, but the same | |
17 | C...kinematics is not relevant e.g. for jet production) and, for the | |
18 | C...'MSbar' fits, in the addition of a Cgamma term related to the | |
19 | C...separation of direct processes. Schematically: | |
20 | C...PDF = VMD (rho, omega, phi) + anomalous (d, u, s, c, b). | |
21 | C...F2 = VMD (rho, omega, phi) + anomalous (d, u, s) + | |
22 | C... Bethe-Heitler (c, b) (+ Cgamma (d, u, s)). | |
23 | C...The J/psi and Upsilon states have not been included in the VMD sum, | |
24 | C...but low c and b masses in the other components should compensate | |
25 | C...for this in a duality sense. | |
26 | ||
27 | C...The calling sequence is the following: | |
28 | C CALL SASGAM1(ISET,X,Q2,P2,F2GM,XPDFGM) | |
29 | C...with the following declaration statement: | |
30 | C DIMENSION XPDFGM(-6:6) | |
31 | C...and, optionally, further information in: | |
32 | C COMMON/SASCOM/XPVMD(-6:6),XPANL(-6:6),XPANH(-6:6),XPBEH(-6:6), | |
33 | C &XPDIR(-6:6) | |
34 | C...Input: ISET = 1 : SaS set 1D ('DIS', Q0 = 0.6 GeV) | |
35 | C = 2 : SaS set 1M ('MSbar', Q0 = 0.6 GeV) | |
36 | C = 3 : SaS set 2D ('DIS', Q0 = 2 GeV) | |
37 | C = 4 : SaS set 2M ('MSbar', Q0 = 2 GeV) | |
38 | C X : x value. | |
39 | C Q2 : Q2 value. | |
40 | C P2 : P2 value; should be = 0. for an on-shell photon. | |
41 | C...Output: F2GM : F2 value of the photon (including factors of alpha_em). | |
42 | C XPFDGM : x times parton distribution functions of the photon, | |
43 | C with elements 0 = g, 1 = d, 2 = u, 3 = s, 4 = c, 5 = b, | |
44 | C 6 = t (always empty!), - for antiquarks (result is same). | |
45 | C...The breakdown by component is stored in the commonblock SASCOM, | |
46 | C with elements as above. | |
47 | C XPVMD : rho, omega, phi VMD part only of output. | |
48 | C XPANL : d, u, s anomalous part only of output. | |
49 | C XPANH : c, b anomalous part only of output. | |
50 | C XPBEH : c, b Bethe-Heitler part only of output. | |
51 | C XPDIR : Cgamma (direct contribution) part only of output. | |
52 | ||
53 | SUBROUTINE SASGAM1(ISET,X,Q2,P2,F2GM,XPDFGM) | |
54 | C...Purpose: to construct the F2 and parton distributions of the photon | |
55 | C...by summing homogeneous (VMD) and inhomogeneous (anomalous) terms. | |
56 | C...For F2, c and b are included by the Bethe-Heitler formula; | |
57 | C...in the 'MSbar' scheme additionally a Cgamma term is added. | |
58 | DIMENSION XPDFGM(-6:6) | |
59 | COMMON/SASCOM/XPVMD(-6:6),XPANL(-6:6),XPANH(-6:6),XPBEH(-6:6), | |
60 | &XPDIR(-6:6) | |
61 | SAVE /SASCOM/ | |
62 | ||
63 | C...Temporary array. | |
64 | DIMENSION XPGA(-6:6) | |
65 | C...Charm and bottom masses (low to compensate for J/psi etc.). | |
66 | DATA PMC/1.3/, PMB/4.6/ | |
67 | C...alpha_em and alpha_em/(2*pi). | |
68 | DATA AEM/0.007297/, AEM2PI/0.0011614/ | |
69 | C...Lambda value for 4 flavours. | |
70 | DATA ALAM/0.20/ | |
71 | C...Mixture u/(u+d), = 0.5 for incoherent and = 0.8 for coherent sum. | |
72 | DATA FRACU/0.8/ | |
73 | C...VMD couplings f_V**2/(4*pi). | |
74 | DATA FRHO/2.20/, FOMEGA/23.6/, FPHI/18.4/ | |
75 | C...Masses for rho (=omega) and phi. | |
76 | DATA PMRHO/0.770/, PMPHI/1.020/ | |
77 | ||
78 | C...Reset output. | |
79 | F2GM=0. | |
80 | DO 100 KFL=-6,6 | |
81 | XPDFGM(KFL)=0. | |
82 | XPVMD(KFL)=0. | |
83 | XPANL(KFL)=0. | |
84 | XPANH(KFL)=0. | |
85 | XPBEH(KFL)=0. | |
86 | XPDIR(KFL)=0. | |
87 | 100 CONTINUE | |
88 | ||
89 | C...Check that input sensible. | |
90 | IF(ISET.LE.0.OR.ISET.GE.5) THEN | |
91 | WRITE(*,*) ' FATAL ERROR: SaSgam called for unknown set' | |
92 | WRITE(*,*) ' ISET = ',ISET | |
93 | STOP | |
94 | ENDIF | |
95 | IF(X.LE.0..OR.X.GT.1.) THEN | |
96 | WRITE(*,*) ' FATAL ERROR: SaSgam called for unphysical x' | |
97 | WRITE(*,*) ' X = ',X | |
98 | STOP | |
99 | ENDIF | |
100 | ||
101 | C...Set Q0 cut-off parameter as function of set used. | |
102 | IF(ISET.LE.2) THEN | |
103 | Q0=0.6 | |
104 | ELSE | |
105 | Q0=2. | |
106 | ENDIF | |
107 | Q02 = Q0**2 | |
108 | ||
109 | C...Call VMD parametrization for d quark and use to give rho, omega, phi. | |
110 | C...Note scale choice and dipole dampening for off-shell photon. | |
111 | P2MX=MAX(P2,Q02) | |
112 | CALL SASVM1(ISET,1,X,Q2,P2MX,ALAM,XPGA) | |
113 | XFVAL=XPGA(1)-XPGA(2) | |
114 | XPGA(1)=XPGA(2) | |
115 | XPGA(-1)=XPGA(-2) | |
116 | FACUD=AEM*(1./FRHO+1./FOMEGA)*(PMRHO**2/(PMRHO**2+P2))**2 | |
117 | FACS=AEM*(1./FPHI)*(PMPHI**2/(PMPHI**2+P2))**2 | |
118 | DO 110 KFL=-5,5 | |
119 | XPVMD(KFL)=(FACUD+FACS)*XPGA(KFL) | |
120 | 110 CONTINUE | |
121 | XPVMD(1)=XPVMD(1)+(1.-FRACU)*FACUD*XFVAL | |
122 | XPVMD(2)=XPVMD(2)+FRACU*FACUD*XFVAL | |
123 | XPVMD(3)=XPVMD(3)+FACS*XFVAL | |
124 | XPVMD(-1)=XPVMD(-1)+(1.-FRACU)*FACUD*XFVAL | |
125 | XPVMD(-2)=XPVMD(-2)+FRACU*FACUD*XFVAL | |
126 | XPVMD(-3)=XPVMD(-3)+FACS*XFVAL | |
127 | ||
128 | C...Call anomalous parametrization for d + u + s. | |
129 | CALL SASAN1(-3,X,Q2,P2MX,ALAM,XPGA) | |
130 | DO 120 KFL=-5,5 | |
131 | XPANL(KFL)=XPGA(KFL) | |
132 | 120 CONTINUE | |
133 | ||
134 | C...Call anomalous parametrization for c and b. | |
135 | CALL SASAN1(4,X,Q2,P2MX,ALAM,XPGA) | |
136 | DO 130 KFL=-5,5 | |
137 | XPANH(KFL)=XPGA(KFL) | |
138 | 130 CONTINUE | |
139 | CALL SASAN1(5,X,Q2,P2MX,ALAM,XPGA) | |
140 | DO 140 KFL=-5,5 | |
141 | XPANH(KFL)=XPANH(KFL)+XPGA(KFL) | |
142 | 140 CONTINUE | |
143 | ||
144 | C...Call Bethe-Heitler term expression for charm and bottom. | |
145 | CALL SASBEH(4,X,Q2,P2,PMC**2,XPBH) | |
146 | XPBEH(4)=XPBH | |
147 | XPBEH(-4)=XPBH | |
148 | CALL SASBEH(5,X,Q2,P2,PMB**2,XPBH) | |
149 | XPBEH(5)=XPBH | |
150 | XPBEH(-5)=XPBH | |
151 | ||
152 | C...For MSbar subtraction call C^gamma term expression for d, u, s. | |
153 | IF(ISET.EQ.2.OR.ISET.EQ.4) THEN | |
154 | CALL SASDIR(X,Q2,P2,Q02,XPGA) | |
155 | DO 150 KFL=-5,5 | |
156 | XPDIR(KFL)=XPGA(KFL) | |
157 | 150 CONTINUE | |
158 | ENDIF | |
159 | ||
160 | C...Store result in output array. | |
161 | DO 160 KFL=-5,5 | |
162 | CHSQ=1./9. | |
163 | IF(IABS(KFL).EQ.2.OR.IABS(KFL).EQ.4) CHSQ=4./9. | |
164 | XPF2=XPVMD(KFL)+XPANL(KFL)+XPBEH(KFL)+XPDIR(KFL) | |
165 | IF(KFL.NE.0) F2GM=F2GM+CHSQ*XPF2 | |
166 | XPDFGM(KFL)=XPVMD(KFL)+XPANL(KFL)+XPANH(KFL) | |
167 | 160 CONTINUE | |
168 | ||
169 | RETURN | |
170 | END |