Simulations of photo-nuclear dijets with Pythia 8 and their sensitivity to nuclear PDFs

Abstract: In ultra-peripheral heavy-ion collisions the nuclei pass with such a large impact parameter that no strong interaction can take place. However, a photon emitted by one nucleus can still interact either with the other nucleus or with a photon from the another nucleus. The former event type corresponds to photoproduction, earlier studied e.g. in electron-proton collisions at HERA, but now with a nuclear target. This provides an unique opportunity to study the nuclear modifications of the PDFs with the photo-nuclear processes measured in lead-lead collisions at the LHC. Here we present our recent implementation of photoproduction processes in the Pythia 8 general-purpose Monte-Carlo event generator. The resulting simulations are compared to charged-hadron and dijet photoproduction data from the HERA experiments, and the theoretical uncertainties of the framework are studied. We discuss how the relevant part of the photon flux from heavy ions is obtained using the equivalent photon approximation and present predictions for photo-nuclear dijet cross section at the LHC. To estimate the potential of this data to further constrain the nuclear PDFs, the expected statistical uncertainty based on LHC luminosity is compared to uncertainties in the current nuclear PDF analyses.