Hadron-ion collisions in Pythia and the vector-meson dominance model for photoproduction

TL;DR

This paper extends the Pythia event generator to simulate hadron-ion collisions with a new vector-meson dominance model for photoproduction, validated against experimental data and applied to ultra-peripheral heavy-ion collisions.

Contribution

The work introduces a technical extension to Pythia for hadron-ion simulations and implements a VMD model for photon interactions, enabling studies of ultra-peripheral collisions and cosmic ray air showers.

Findings

VMD model agrees with HERA photoproduction data

Simulated ultra-peripheral collisions match LHC measurements

Azimuthal anisotropies are consistent with ATLAS results

Abstract

We present an extension to the Pythia Monte Carlo event generator that enables simulations of collisions between a generic hadron beam on a nuclear target with energy variation in event-by-event basis. This builds upon Pythia's module for heavy ions, Angantyr, as well as previous work on simulating hadron-proton collisions. As such, the extensions in this work are largely technical, except for a rudimentary model for hadronic fluctuations. With hadron-ion simulations, we implement an explicit vector-meson dominance (VMD) model that can be used to simulate interactions of hadronic component of real photons in photo-nuclear collisions. Such processes can be studied in ultra-peripheral heavy-ion collisions and in the future also with the upcoming Electron-Ion Collider. Our work also has applications to hadronic showers, e.g. air showers initiated by high-energy cosmic rays. We first validate the VMD model by comparing to HERA photoproduction data on proton target. Then we apply this to generate events for ultra-peripheral heavy-ion collisions at the LHC and present the results corresponding to the event-selection criteria matching to a recent ATLAS analysis. We find that single-particle multiplicity and rapidity distributions are well in line with the measured ones. We also construct the Fourier coefficients from two-particle correlations for the simulated events and study whether the resulting azimuthal anisotropies are consistent with the ATLAS results.