pystga.F

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      SUBROUTINE pystga(X,Q2,XPGA)
 
C...Gives photon structure function.
      common/ludat1/mstu(200),paru(200),mstj(200),parj(200)
      common/pypars/mstp(200),parp(200),msti(200),pari(200)
      common/pyint1/mint(400),vint(400)
      SAVE /ludat1/
      SAVE /pypars/,/pyint1/
      dimension xpga(-6:6),dgag(4,3),dgbg(4,3),dgcg(4,3),dgan(4,3),
     &dgbn(4,3),dgcn(4,3),dgdn(4,3),dgen(4,3),dgas(4,3),dgbs(4,3),
     &dgcs(4,3),dgds(4,3),dges(4,3)
 
C...The following data lines are coefficients needed in the
C...Drees and Grassie photon structure function parametrization.
      DATA dgag/-.207e0,.6158e0,1.074e0,0.e0,.8926e-2,.6594e0,
     &.4766e0,.1975e-1,.03197e0,1.018e0,.2461e0,.2707e-1/
      DATA dgbg/-.1987e0,.6257e0,8.352e0,5.024e0,.5085e-1,.2774e0,
     &-.3906e0,-.3212e0,-.618e-2,.9476e0,-.6094e0,-.1067e-1/
      DATA dgcg/5.119e0,-.2752e0,-6.993e0,2.298e0,-.2313e0,.1382e0,
     &6.542e0,.5162e0,-.1216e0,.9047e0,2.653e0,.2003e-2/
      DATA dgan/2.285e0,-.1526e-1,1330.e0,4.219e0,-.3711e0,1.061e0,
     &4.758e0,-.1503e-1,15.8e0,-.9464e0,-.5e0,-.2118e0/
      DATA dgbn/6.073e0,-.8132e0,-41.31e0,3.165e0,-.1717e0,.7815e0,
     &1.535e0,.7067e-2,2.742e0,-.7332e0,.7148e0,3.287e0/
      DATA dgcn/-.4202e0,.1778e-1,.9216e0,.18e0,.8766e-1,.2197e-1,
     &.1096e0,.204e0,.2917e-1,.4657e-1,.1785e0,.4811e-1/
      DATA dgdn/-.8083e-1,.6346e0,1.208e0,.203e0,-.8915e0,.2857e0,
     &2.973e0,.1185e0,-.342e-1,.7196e0,.7338e0,.8139e-1/
      DATA dgen/.5526e-1,1.136e0,.9512e0,.1163e-1,-.1816e0,.5866e0,
     &2.421e0,.4059e0,-.2302e-1,.9229e0,.5873e0,-.79e-4/
      DATA dgas/16.69e0,-.7916e0,1099.e0,4.428e0,-.1207e0,1.071e0,
     &1.977e0,-.8625e-2,6.734e0,-1.008e0,-.8594e-1,.7625e-1/
      DATA dgbs/.176e0,.4794e-1,1.047e0,.25e-1,25.e0,-1.648e0,
     &-.1563e-1,6.438e0,59.88e0,-2.983e0,4.48e0,.9686e0/
      DATA dgcs/-.208e-1,.3386e-2,4.853e0,.8404e0,-.123e-1,1.162e0,
     &.4824e0,-.11e-1,-.3226e-2,.8432e0,.3616e0,.1383e-2/
      DATA dgds/-.1685e-1,1.353e0,1.426e0,1.239e0,-.9194e-1,.7912e0,
     &.6397e0,2.327e0,-.3321e-1,.9475e0,-.3198e0,.2132e-1/
      DATA dges/-.1986e0,1.1e0,1.136e0,-.2779e0,.2015e-1,.9869e0,
     &-.7036e-1,.1694e-1,.1059e0,.6954e0,-.6663e0,.3683e0/
 
C...Photon structure function from Drees and Grassie.
C...Allowed variable range: 1 GeV^2 < Q^2 < 10000 GeV^2.
      DO 100 kfl=-6,6
      xpga(kfl)=0.
  100 CONTINUE
      vint(231)=1.
      IF(mstp(57).LE.0) THEN
        t=log(1./0.16)
      ELSE
        t=log(min(1e4,max(1.,q2))/0.16)
      ENDIF
      x1=1.-x
      nf=3
      IF(q2.GT.25.) nf=4
      IF(q2.GT.300.) nf=5
      nfe=nf-2
      aem=paru(101)
 
C...Evaluate gluon content.
      dga=dgag(1,nfe)*t**dgag(2,nfe)+dgag(3,nfe)*t**(-dgag(4,nfe))
      dgb=dgbg(1,nfe)*t**dgbg(2,nfe)+dgbg(3,nfe)*t**(-dgbg(4,nfe))
      dgc=dgcg(1,nfe)*t**dgcg(2,nfe)+dgcg(3,nfe)*t**(-dgcg(4,nfe))
      xpgl=dga*x**dgb*x1**dgc
 
C...Evaluate up- and down-type quark content.
      dga=dgan(1,nfe)*t**dgan(2,nfe)+dgan(3,nfe)*t**(-dgan(4,nfe))
      dgb=dgbn(1,nfe)*t**dgbn(2,nfe)+dgbn(3,nfe)*t**(-dgbn(4,nfe))
      dgc=dgcn(1,nfe)*t**dgcn(2,nfe)+dgcn(3,nfe)*t**(-dgcn(4,nfe))
      dgd=dgdn(1,nfe)*t**dgdn(2,nfe)+dgdn(3,nfe)*t**(-dgdn(4,nfe))
      dge=dgen(1,nfe)*t**dgen(2,nfe)+dgen(3,nfe)*t**(-dgen(4,nfe))
      xpqn=x*(x**2+x1**2)/(dga-dgb*log(x1))+dgc*x**dgd*x1**dge
      dga=dgas(1,nfe)*t**dgas(2,nfe)+dgas(3,nfe)*t**(-dgas(4,nfe))
      dgb=dgbs(1,nfe)*t**dgbs(2,nfe)+dgbs(3,nfe)*t**(-dgbs(4,nfe))
      dgc=dgcs(1,nfe)*t**dgcs(2,nfe)+dgcs(3,nfe)*t**(-dgcs(4,nfe))
      dgd=dgds(1,nfe)*t**dgds(2,nfe)+dgds(3,nfe)*t**(-dgds(4,nfe))
      dge=dges(1,nfe)*t**dges(2,nfe)+dges(3,nfe)*t**(-dges(4,nfe))
      dgf=9.
      IF(nf.EQ.4) dgf=10.
      IF(nf.EQ.5) dgf=55./6.
      xpqs=dgf*x*(x**2+x1**2)/(dga-dgb*log(x1))+dgc*x**dgd*x1**dge
      IF(nf.LE.3) THEN
        xpqu=(xpqs+9.*xpqn)/6.
        xpqd=(xpqs-4.5*xpqn)/6.
      ELSEIF(nf.EQ.4) THEN
        xpqu=(xpqs+6.*xpqn)/8.
        xpqd=(xpqs-6.*xpqn)/8.
      ELSE
        xpqu=(xpqs+7.5*xpqn)/10.
        xpqd=(xpqs-5.*xpqn)/10.
      ENDIF
 
C...Put into output arrays.
      xpga(0)=aem*xpgl
      xpga(1)=aem*xpqd
      xpga(2)=aem*xpqu
      xpga(3)=aem*xpqd
      IF(nf.GE.4) xpga(4)=aem*xpqu
      IF(nf.GE.5) xpga(5)=aem*xpqd
      DO 110 kfl=1,6
      xpga(-kfl)=xpga(kfl)
  110 CONTINUE
 
      RETURN
      END