Spatial resolution of dijet photoproduction in near-encounter ultraperipheral nuclear collisions

We present next-to-leading order perturbative QCD predictions for inclusive dijet photoproduction in ultraperipheral nucleus-nucleus collisions (UPCs) within the impact-parameter dependent equivalent photon approximation. Taking into account the finite size of both the photon-emitting nucleus and the target nucleus, we show that this process is sensitive to the transverse-plane geometry of the UPC events. We show that this leads to a sizable, 20–40% effect for large values of the <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>z</a:mi><a:mi>γ</a:mi></a:msub></a:math> variable in the dijet photoproduction cross section in lead-lead UPCs at 5.02 TeV compared to the widely used pointlike approximation where the nuclear radius is accounted for only as a sharp cutoff in the photon flux calculation. This resolution of the spatial degrees of freedom is a result of having high-transverse-momentum jets in the final state, which at the large-<b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>z</b:mi><b:mi>γ</b:mi></b:msub></b:math> kinematics requires a highly energetic photon in the initial state, thus biasing the collisions to small impact-parameter “near-encounter” configurations. We further discuss the role of the forward-neutron event-class selection in isolating the photonuclear cross section in the nucleus-nucleus collisions, and employ the needed electromagnetic breakup survival factor in our predictions. Published by the American Physical Society 2024