pythiaeRHIC/pythia/pypcmp.f

0001
0002 C*********************************************************************
0003
0004 C...PYPCMP: Auxiliary to PYPDFU.
0005 C...Giving the momentum integral of a companion quark, with its
0006 C...partner at XS, using an approximate gluon density like (1-x)^NPOW/x.
0007 C...The value corresponds to an unrescaled range between 0 and 1-XS.
0008
0009       FUNCTION PYPCMP(XS,NPOW)
0010       IMPLICIT NONE
0011       DOUBLE PRECISION XS, PYPCMP
0012       INTEGER NPOW
0013       IF (XS.GE.1D0.OR.XS.LE.0D0) THEN
0014         PYPCMP=0D0
0015       ELSEIF (NPOW.LE.0) THEN
0016         PYPCMP=XS*(5D0+XS*(-9D0-2D0*XS*(-3D0+XS))+3D0*LOG(XS))
0017         PYPCMP=PYPCMP/((-1D0+XS)*(2D0+XS*(-1D0+2D0*XS)))
0018       ELSEIF (NPOW.EQ.1) THEN
0019         PYPCMP=-1D0-3D0*XS+(2D0*(-1D0+XS)**2*(1D0+XS+XS**2))
0020      &       /(2D0+XS**2*(XS-3D0)+3D0*XS*LOG(XS))
0021       ELSEIF (NPOW.EQ.2) THEN
0022         PYPCMP=XS*((1D0-XS)*(19D0+XS*(43D0+4D0*XS))
0023      &       +6D0*LOG(XS)*(1D0+6D0*XS+4D0*XS**2))
0024         PYPCMP=PYPCMP/(4D0*((XS-1D0)*(1D0+XS*(4D0+XS))
0025      &       -3D0*XS*LOG(XS)*(1+XS)))
0026       ELSEIF (NPOW.EQ.3) THEN
0027         PYPCMP=3D0*XS*((XS-1)*(7D0+XS*(28D0+13D0*XS))
0028      &       -2D0*LOG(XS)*(1D0+XS*(9D0+2D0*XS*(6D0+XS))))
0029         PYPCMP=PYPCMP/(4D0+27D0*XS-31D0*XS**3
0030      &       +6D0*XS*LOG(XS)*(3D0+2D0*XS*(3D0+XS)))
0031       ELSE
0032         PYPCMP=(-9D0*XS*(XS**2-1D0)*(5D0+XS*(24D0+XS))+12D0*XS*LOG(XS)
0033      &       *(1D0+2D0*XS)*(1D0+2D0*XS*(5D0+2D0*XS)))
0034         PYPCMP=PYPCMP/(8D0*(1D0+2D0*XS)*((XS-1D0)*(1D0+XS*(10D0+XS))
0035      &       -6D0*XS*LOG(XS)*(1D0+XS)))
0036       ENDIF
0037       RETURN
0038       END