Photo-nuclear collisions in Pythia 8

Abstract

We present a new extension in Pythia Monte Carlo event generator that allows to simulate the leading contribution in collisions of a real photon and a heavy ion. The model is based on a vector meson dominance (VMD) where a real photon is modelled as a linear combination of different vector-meson states, and an extension of Angantyr, the heavy-ion model in Pythia, that allow simulations of a generic hadron colliding with an ion target. We first verify the VMD implementation by comparing simulations to HERA photoproduction data. Then we present simulated results corresponding to event selection criteria applied in a recent ATLAS analysis for ultra-peripheral Pb-Pb collisions at the LHC. We find that the simulated results are in line with the ATLAS data when accounting for the limited detector efficiency for charged-particle reconstruction. We also consider two-particle correlations and study whether the simulated events reproduce the collective behaviour seen in the ATLAS photon-lead data.