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Changes to PYTHIA

interface to call nPDF with LHAPDF support

  • code revisions
  1. uncomment "CALL PDFSET(PARM,VALUE)" to link to LHAPDF.
  2. "COMMON/PYNUCL/ INUMOD,CHANUM,ORDER" global variables to be initialized in the main function and used in PYPDFU routine. INUMOD and CHANUM specifies the A and Z for a nuclear type, respectively. ORDER=order*100+ErrSet, if order=1, LO nPDF; if order=2, NLO nPDF; ErrSet specifies the error set for EPS09. nPDF will be in effect if INUMOD>0 and ORDER>100.
  3. when compiling the code, the executable file needs to be linked to the LHAPDF library
  • input parameter revisions
  1. nPDF parameter 1 line: nulcear Mass number A, charge number Z. A<=1 no nPDF
  2. nPDF parameter 2 line: ORDER = order*100+ErrSet. order=1, LO nPDF; 2, NLO nPDF; 0, no nPDF; ErrSet error set index.

Kinematic calculation when sqrt(s) is close to proton mass

  • code revisions

changes made to PYGAGA routine for xBj and W2 calculation
Originally: xBj=Q2/sy, W2= m2p-Q2+Q2/xBj
change to:
xBj=Q2/(2mp nu), W2= m2p-Q2+2mp nu, nu is obtained as nu = EeTRF y

intrinsic kT with a double gaussian form

  • code revisions

changes made to PYREMN and PYDISG rountines when sampling intrinsic kT. Calculation illustrated as:
d2NdkT 2= exp(kT 2/B1)+r*exp(kT 2/B2)
Based on the r value sampled, the intrinsic kT value will be set according to B1 width or B2 width, respectively.

  • input parameter revisions
  1. MSTP(91)=5 to activate this option
  2. B1=PARP(91)*PARP(91), width of the first gaussian (default intrinsic kT width)
  3. B2=PARP(195)*PARP(195), width of the second gaussian
  4. r=PARP(196), fraction changing the mix of two gaussian forms