2024 Theses Doctoral

Photoproduction Processes as a Probe of the Strong Nuclear Force in Relativistic Heavy Ion Collisions at the LHC

Gilbert, Benjamin Jacob

The intense electromagnetic fields surrounding highly charged, ultra-relativistic ions at the LHC provide an intense flux of quasi-real photons. This high-energy photon flux provides a precise probe of the structure of the nucleus in photonuclear interactions and a tool to precisely study the properties of photoproduction in two-photon interactions.

This thesis presents recent results from studies of photoproduction processes in √𝒔_𝐍𝐍 = 5.02 TeV Pb+Pb collisions at the LHC, using data collected in 2015 and 2018. A measurement of the differential cross-section for photonuclear jet production is performed with an integrated luminosity of 1.72 nb⁻¹. This measurement is triple-differential in two different sets of kinematic variables ([𝐻_𝐓, 𝒙_𝐀, 𝒛_𝛾] and [𝐻_𝐓, 𝒚_jets, 𝒎_jets) which allow for a direct measurement of the kinematics of struck partons in the nuclear target, and it is fully unfolded in three dimensions.

After performing detailed studies of the rate for nuclear breakup in these collisions, the photonuclear jet cross-sections agree with leading-order predictions at the level of 10%. The uncertainty on this measurement and full treatment of its correlated uncertainties will allow for it to significantly constrain the nuclear parton distributions over a wide region of parton kinematics. A measurement was also performed of dimuon production via two-photon fusion in Pb+Pb collisions with nuclear overlap. This measurement studied the relative deflection of muons in each pair, and its results indicated that these distributions are substantially modified as a function of collision centrality. This modification is mostly well-described by initial-state models, which improve the modelling of the photon flux by incorporating correlations between the collision impact parameter and photon 𝒑_𝐓. Additional studies were performed to constrain the potential impact of any deflections due to the presence of strong magnetic fields in the hot, dense medium produced in heavy ion collisions.