pypdga.f

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C*********************************************************************
 
C...PYPDGA
C...Gives photon parton distribution.
 
      SUBROUTINE pypdga(X,Q2,XPGA)
 
C...Double precision and integer declarations.
      IMPLICIT DOUBLE PRECISION(a-h, o-z)
      IMPLICIT INTEGER(i-n)
      INTEGER pyk,pychge,pycomp
C...Commonblocks.
      common/pydat1/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 /pydat1/,/pypars/,/pyint1/
C...Local arrays.
      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 parton distribution parametrization.
      DATA dgag/-.207d0,.6158d0,1.074d0,0.d0,.8926d-2,.6594d0,
     &.4766d0,.1975d-1,.03197d0,1.018d0,.2461d0,.2707d-1/
      DATA dgbg/-.1987d0,.6257d0,8.352d0,5.024d0,.5085d-1,.2774d0,
     &-.3906d0,-.3212d0,-.618d-2,.9476d0,-.6094d0,-.1067d-1/
      DATA dgcg/5.119d0,-.2752d0,-6.993d0,2.298d0,-.2313d0,.1382d0,
     &6.542d0,.5162d0,-.1216d0,.9047d0,2.653d0,.2003d-2/
      DATA dgan/2.285d0,-.1526d-1,1330.d0,4.219d0,-.3711d0,1.061d0,
     &4.758d0,-.1503d-1,15.8d0,-.9464d0,-.5d0,-.2118d0/
      DATA dgbn/6.073d0,-.8132d0,-41.31d0,3.165d0,-.1717d0,.7815d0,
     &1.535d0,.7067d-2,2.742d0,-.7332d0,.7148d0,3.287d0/
      DATA dgcn/-.4202d0,.1778d-1,.9216d0,.18d0,.8766d-1,.2197d-1,
     &.1096d0,.204d0,.2917d-1,.4657d-1,.1785d0,.4811d-1/
      DATA dgdn/-.8083d-1,.6346d0,1.208d0,.203d0,-.8915d0,.2857d0,
     &2.973d0,.1185d0,-.342d-1,.7196d0,.7338d0,.8139d-1/
      DATA dgen/.5526d-1,1.136d0,.9512d0,.1163d-1,-.1816d0,.5866d0,
     &2.421d0,.4059d0,-.2302d-1,.9229d0,.5873d0,-.79d-4/
      DATA dgas/16.69d0,-.7916d0,1099.d0,4.428d0,-.1207d0,1.071d0,
     &1.977d0,-.8625d-2,6.734d0,-1.008d0,-.8594d-1,.7625d-1/
      DATA dgbs/.176d0,.4794d-1,1.047d0,.25d-1,25.d0,-1.648d0,
     &-.1563d-1,6.438d0,59.88d0,-2.983d0,4.48d0,.9686d0/
      DATA dgcs/-.208d-1,.3386d-2,4.853d0,.8404d0,-.123d-1,1.162d0,
     &.4824d0,-.11d-1,-.3226d-2,.8432d0,.3616d0,.1383d-2/
      DATA dgds/-.1685d-1,1.353d0,1.426d0,1.239d0,-.9194d-1,.7912d0,
     &.6397d0,2.327d0,-.3321d-1,.9475d0,-.3198d0,.2132d-1/
      DATA dges/-.1986d0,1.1d0,1.136d0,-.2779d0,.2015d-1,.9869d0,
     &-.7036d-1,.1694d-1,.1059d0,.6954d0,-.6663d0,.3683d0/
 
C...Photon parton distribution from Drees and Grassie.
C...Allowed variable range: 1 GeV^2 < Q^2 < 10000 GeV^2.
      DO 100 kfl=-6,6
        xpga(kfl)=0d0
  100 CONTINUE
      vint(231)=1d0
      IF(mstp(57).LE.0) THEN
        t=log(1d0/0.16d0)
      ELSE
        t=log(min(1d4,max(1d0,q2))/0.16d0)
      ENDIF
      x1=1d0-x
      nf=3
      IF(q2.GT.25d0) nf=4
      IF(q2.GT.300d0) 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=9d0
      IF(nf.EQ.4) dgf=10d0
      IF(nf.EQ.5) dgf=55d0/6d0
      xpqs=dgf*x*(x**2+x1**2)/(dga-dgb*log(x1))+dgc*x**dgd*x1**dge
      IF(nf.LE.3) THEN
        xpqu=(xpqs+9d0*xpqn)/6d0
        xpqd=(xpqs-4.5d0*xpqn)/6d0
      ELSEIF(nf.EQ.4) THEN
        xpqu=(xpqs+6d0*xpqn)/8d0
        xpqd=(xpqs-6d0*xpqn)/8d0
      ELSE
        xpqu=(xpqs+7.5d0*xpqn)/10d0
        xpqd=(xpqs-5d0*xpqn)/10d0
      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