Dijet photoproduction and transverse-plane geometry in ultra-peripheral nucleus-nucleus collisions

TL;DR

This paper provides advanced NLO pQCD predictions for dijet photoproduction in ultra-peripheral lead-lead collisions, highlighting the impact of collision geometry and forward-neutron event modeling on the cross section.

Contribution

It introduces the first NLO inclusive UPC dijet predictions that incorporate impact-parameter dependent photon flux and forward-neutron event class modeling, linking collision geometry to observable outcomes.

Findings

Dijet photoproduction is sensitive to the transverse-plane collision geometry.

Impact-parameter dependence affects the cross section at forward rapidities.

Modeling of forward-neutron events improves agreement with experimental measurements.

Abstract

We present new NLO pQCD predictions for the inclusive photoproduction of dijets in ultra-peripheral (UPC) lead-lead collisions at 5.02 TeV with a realistic impact-parameter dependent effective photon flux obtained through the Woods-Saxon nuclear profile. For the first time in NLO inclusive UPC dijet predictions, we take into account also the modelling of the forward-neutron event class required in the experimental measurements. We show that since the dijet photoproduction at forward rapidities requires an energetic photon in the initial state, this biases the cross section to be dominated by events with relatively small impact parameters between the nuclei, of the order of a few nuclear radii. This leads to a sensitivity to the transverse-plane collision geometry, which we take properly into account by including effects from the finite extent of both the photon-emitting and the target nucleus. We also test the potential sensitivity to the spatial dependence of nuclear parton distribution functions in connection with this finding.