Photoproduction in general-purpose event generators

TL;DR

This paper compares three general-purpose Monte Carlo event generators for jet photoproduction in $e^+e^-$ and $ep$ collisions, analyzing their differences and agreement with experimental data, and providing predictions for the EIC.

Contribution

It systematically analyzes and quantifies differences between HERWIG, PYTHIA, and SHERPA in photoproduction processes, highlighting areas for improvement and future research.

Findings

All generators describe the data within uncertainties.

PYTHIA and SHERPA provide particularly good descriptions.

Predictions for the EIC suggest the need for a global photon PDF refit.

Abstract

We compare the three general-purpose Monte Carlo event generators, HERWIG, PYTHIA, and SHEPRA for jet photoproduction processes in $e^+e^-$ and $ep$ collisions. Due to the lower energy scales probed, photoproduction is particularly sensitive to non-perturbative corrections. In a systematic analysis we disentangle and quantify the differences between the generators in these processes, i.e. contributions from beam remnants, parton showers, multiparton interactions (MPIs), and hadronisation modelling. We outline the default inputs and implementation differences and compare the computations with experimental data from LEP and HERA. We find that all generators provide a decent description of the data within the uncertainties, with particularly good descriptions by the LO-accurate PYTHIA and the NLO-accurate SHERPA. Finally, we also present predictions for the upcoming EIC for jet observables and event shapes and conclude that a modern global refit of the photon parton distributions and dedicated experimental measurements ported to the RIVET framework to constrain non-perturbative parameters are the key prerequisites for precision photoproduction phenomenology at the EIC.