gphoton.f

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C
C
C
        FUNCTION gphoton(Y1,Y2,PT2)
        IMPLICIT REAL*8 (a-h,o-z)
        REAL hipr1(100),hint1(100)
        common/hiparnt/hipr1,ihpr2(50),hint1,ihnt2(50)
        SAVE  /hiparnt/
        dimension f(2,7)
        xt=2.0*dsqrt(pt2)/hint1(1)
        x1=0.50*xt*(dexp(y1)+dexp(y2))
        x2=0.50*xt*(dexp(-y1)+dexp(-y2))
        z=dsqrt(1.d0-xt**2/x1/x2)
        ss=x1*x2*hint1(1)**2
        t=-(1.0-z)/2.0
        u=-(1.0+z)/2.0
        af=3.0
        dlam=hipr1(15)
        aph=12.0*3.1415926/(33.0-2.0*af)/dlog(pt2/dlam**2)
        aphem=1.0/137.0
C
        CALL parton(f,x1,x2,pt2)
C
        g11=-(u**2+1.0)/u/3.0*f(1,7)*(4.0*f(2,1)+4.0*f(2,2)
     &      +f(2,3)+f(2,4)+f(2,5)+f(2,6))/9.0
        g12=-(t**2+1.0)/t/3.0*f(2,7)*(4.0*f(1,1)+4.0*f(1,2)
     &      +f(1,3)+f(1,4)+f(1,5)+f(1,6))/9.0
        g2=8.0*(u**2+t**2)/u/t/9.0*(4.0*f(1,1)*f(2,2)
     &     +4.0*f(1,2)*f(2,1)+f(1,3)*f(2,4)+f(1,4)*f(2,3)
     &     +f(1,5)*f(2,6)+f(1,6)*f(2,5))/9.0
C
        gphoton=(g11+g12+g2)*hipr1(23)*3.14159*aph*aphem/ss**2
        RETURN
        END