hijing/src/gphoton.f

0001 C
0002 C
0003 C
0004         FUNCTION GPHOTON(Y1,Y2,PT2)
0005         IMPLICIT REAL*8 (A-H,O-Z)
0006         REAL HIPR1(100),HINT1(100)
0007         COMMON/HIPARNT/HIPR1,IHPR2(50),HINT1,IHNT2(50)
0008         SAVE  /HIPARNT/
0009         DIMENSION F(2,7)
0010         XT=2.0*DSQRT(PT2)/HINT1(1)
0011         X1=0.50*XT*(DEXP(Y1)+DEXP(Y2))
0012         X2=0.50*XT*(DEXP(-Y1)+DEXP(-Y2))
0013         Z=DSQRT(1.D0-XT**2/X1/X2)
0014         SS=X1*X2*HINT1(1)**2
0015         T=-(1.0-Z)/2.0
0016         U=-(1.0+Z)/2.0
0017         AF=3.0
0018         DLAM=HIPR1(15)
0019         APH=12.0*3.1415926/(33.0-2.0*AF)/DLOG(PT2/DLAM**2)
0020         APHEM=1.0/137.0
0021 C
0022         CALL PARTON(F,X1,X2,PT2)
0023 C
0024         G11=-(U**2+1.0)/U/3.0*F(1,7)*(4.0*F(2,1)+4.0*F(2,2)
0025      &      +F(2,3)+F(2,4)+F(2,5)+F(2,6))/9.0
0026         G12=-(T**2+1.0)/T/3.0*F(2,7)*(4.0*F(1,1)+4.0*F(1,2)
0027      &      +F(1,3)+F(1,4)+F(1,5)+F(1,6))/9.0
0028         G2=8.0*(U**2+T**2)/U/T/9.0*(4.0*F(1,1)*F(2,2)
0029      &     +4.0*F(1,2)*F(2,1)+F(1,3)*F(2,4)+F(1,4)*F(2,3)
0030      &     +F(1,5)*F(2,6)+F(1,6)*F(2,5))/9.0
0031 C
0032         GPHOTON=(G11+G12+G2)*HIPR1(23)*3.14159*APH*APHEM/SS**2
0033         RETURN
0034         END