PDFxTMDLib usage in KaTie input files

KatieWithPDFxTMDLib is a clone of Katie Parton level event generator. This document explains how to use PDFxTMDLib (TMDs and collinear PDFs), from KaTie input files. Example inputs are in diphoton/:

• diphoton/pdfxtmdKt (TMD / kT-dependent)
• diphoton/pdfxtmdCol (collinear / cPDF)

Setup (library paths)

Edit settings.py and set the PDFxTMDLib path before building:

# Path to the directory where libPDFxTMDLib.so is located.
# Leave empty if PDFxTMD is not available on this machine.
PDFXTMDpath = '/usr/local/lib'

Then generate the library:

./config.py lib

Quick start

1. Build KaTie with PDFxTMDLib linked (see ./config.py lib above).
2. In the input file, set PDFxTMDSet and choose either TMD (default) or cPDF mode.
3. Optionally use PDFxTMDLib for alphaQCD with PDFxTMDCoupling.

Input keywords

Required

• PDFxTMDSet = <setName> <beamA> <beamB> <member>
  • setName: PDFxTMDLib set name (string)
  • beamA, beamB: beam IDs (typically 2212 for proton)
  • member: set member index (integer, often 0)

Optional

• PDFxTMDType = cpdf
  • Use collinear PDFs from PDFxTMDLib.
  • If omitted, KaTie uses PDFxTMD TMDs (kT-dependent) by default.
• PDFxTMDCoupling = yes|no|<setName>
  • If yes, alphaQCD is taken from PDFxTMDLib. If a set name is provided, that set is used for the coupling.
• PDFxTMDCouplingSet = <setName>
  • Explicit set name for alphaQCD (overrides default behavior).
• useLHAPDF = no|yes
  • If no, KaTie will skip LHAPDF initialization and rely on PDFxTMDLib.

Choosing TMD vs collinear (cPDF)

TMD mode (kT-dependent)

Use off-shell beams and do not set PDFxTMDType:

PDFxTMDSet = PB-LO-HERAI+II-2020-set1 2212 2212 0
PDFxTMDCoupling = yes
useLHAPDF = no

offshell = 1 1

Collinear mode (cPDF)

Use on-shell beams and set PDFxTMDType = cpdf:

PDFxTMDSet = MSTW2008lo68cl 2212 2212 0
PDFxTMDType = cpdf
PDFxTMDCoupling = yes
useLHAPDF = no

offshell = 0 0

Notes and tips

• PDFxTMDSet is used for both beams by default. If you need different sets per beam, use the two-set form supported in your workflow (see local examples).
• If you set useLHAPDF = no, KaTie will not call LHAPDF init or LHAPDF PDFs.
• If you set PDFxTMDType = cpdf, KaTie will not initialize PDFxTMD TMDs.

Troubleshooting

• Error: LHAPDF::ReadError Info file not found for PDF set ...
  • Ensure useLHAPDF = no when you only want PDFxTMDLib.
  • Regenerate the run directory after changing input options.
• Still seeing LHAPDF calls in generated code
  • Re-run ./run.sh prepare <input> <dir> so the template is re-expanded.

Example inputs

See:

• diphoton/pdfxtmdKt (TMD)
• diphoton/pdfxtmdCol (cPDF)