Measurement of photonuclear jet production in ultra-peripheral Pb+Pb collisions at s₍₍ = 5. 02 TeV with the ATLAS detector

In ultra-relativistic heavy ion collisions, the photoproduction of high-energy jets can be used to constrain nuclear parton distributions for a wide range of parton kinematics. Results are presented from a measurement of photonuclear production of dijet and multi-jet final states in ultra-peripheral Pb+Pb collisions at s₍₍ = 5. 02 TeV using a data set recorded in 2018 with the ATLAS detector at the LHC and corresponding to an integrated luminosity of 1. 72 nb^-1. Photonuclear final states are selected by requiring a rapidity gap in the photon direction; this selects events where one of the outgoing nuclei remains intact. Jets are reconstructed using the anti-kₜ algorithm with radius parameter, R = 0. 4. Triple-differential cross-sections, unfolded for detector response, are measured and presented using two sets of kinematic variables. The first set consists of the total transverse momentum (HT), rapidity, and mass of the jet system. The second set uses HT and particle-level nuclear and photon parton momentum fractions, xA and z_, respectively. The results are compared with leading-order perturbative QCD calculations of photonuclear jet production cross-sections, demonstrating their potential to provide a strong new constraint on nuclear parton distributions.