Photon Fragmentation Functions in LHAPDF: Existing Sets and a Pythia-Based Extraction

Abstract

Prompt-photon predictions with experimentally relevant isolation criteria retain a residual dependence on parton-to-photon fragmentation functions, whose phenomenological uncertainties remain only weakly constrained. We present a pragmatic study of photon fragmentation functions extracted from Pythia parton-shower events and stored in LHAPDF grids. We briefly review the role of photon fragmentation in perturbative QCD and then construct shower-level estimates of the flavour-dependent functions $D^{\mathrm{PS}}_{\gamma/i}(z,\mu_D)$ as functions of the photon momentum fraction $z$ and fragmentation scale $\mu_D$. The extracted distributions show the expected qualitative hierarchy between quark and gluon channels and an increasing fragmentation contribution with scale. In parallel, we convert the commonly used BFG, GRV, and GDRG parameterizations into LHAPDF grids and study their interpolation. We find that the default logcubic interpolation in LHAPDF reproduces the non-trivial $z$- and scale-dependence of the tabulated fragmentation functions more faithfully than the linear interpolation commonly used in legacy implementations. We also define a simple collider-oriented envelope set, based on the BFG and GRV families. These results provide a portable baseline for future phenomenological studies and for more systematic extractions of photon fragmentation functions.