Measurement of photonuclear jet production in ultra-peripheral Pb+Pb collisions at $\sqrt{s_{\text{NN}}} = 5.02$ TeV with the ATLAS detector

Abstract

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 \mbox{Pb+Pb} collisions at $\sqrt{s_{\text{NN}}} = 5.02$ TeV using a data set recorded in 2018 with the ATLAS detector at the LHC and corresponding to an integrated luminosity of 1.72 $\text{nb}^{-1}$. 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-$k_\text{t}$ 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 ($H_\text{T}$), rapidity, and mass of the jet system. The second set uses $H_\text{T}$ and particle-level nuclear and photon parton momentum fractions, $x_\text{A}$ and $z_{\gamma}$, 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.