pythiaeRHIC/pythia/pyfcmp.f

0001
0002 C*********************************************************************
0003
0004 C...PYFCMP: Auxiliary to PYPDFU and PYPTIS.
0005 C...Giving the x*f pdf of a companion quark, with its partner at XS,
0006 C...using an approximate gluon density like (1-X)^NPOW/X. The value
0007 C...corresponds to an unrescaled range between 0 and 1-X.
0008
0009       FUNCTION PYFCMP(XC,XS,NPOW)
0010       IMPLICIT NONE
0011       DOUBLE PRECISION XC, XS, Y, PYFCMP,FAC
0012       INTEGER NPOW
0013
0014       PYFCMP=0D0
0015 C...Parent gluon momentum fraction
0016       Y=XC+XS
0017       IF (Y.GE.1D0) RETURN
0018 C...Common factor (includes factor XC, since PYFCMP=x*f)
0019       FAC=3D0*XC*XS*(XC**2+XS**2)/(Y**4)
0020 C...Store normalized companion x*f distribution.
0021       IF (NPOW.LE.0) THEN
0022         PYFCMP=FAC/(2D0-XS*(3D0-XS*(3D0-2D0*XS)))
0023       ELSEIF (NPOW.EQ.1) THEN
0024         PYFCMP=FAC*(1D0-Y)/(2D0+XS**2*(-3D0+XS)+3D0*XS*LOG(XS))
0025       ELSEIF (NPOW.EQ.2) THEN
0026         PYFCMP=FAC*(1D0-Y)**2/(2D0*((1D0-XS)*(1D0+XS*(4D0+XS))
0027      &       +3D0*XS*(1D0+XS)*LOG(XS)))
0028       ELSEIF (NPOW.EQ.3) THEN
0029         PYFCMP=FAC*(1D0-Y)**3*2D0/(4D0+27D0*XS-31D0*XS**3
0030      &       +6D0*XS*LOG(XS)*(3D0+2D0*XS*(3D0+XS)))
0031       ELSEIF (NPOW.GE.4) THEN
0032         PYFCMP=FAC*(1D0-Y)**4/(2D0*(1D0+2D0*XS)*((1D0-XS)*(1D0+
0033      &       XS*(10D0+XS))+6D0*XS*LOG(XS)*(1D0+XS)))
0034       ENDIF
0035       RETURN
0036       END