Investigation of rare processes with two photons in the final state in heavy-ion collisions at the LHC

Thesis presents the role of Monte Carlo (MC) generators in studies of ultra-peripheral collisions (UPCs) of heavy-ions (Pb-Pb) at the Large Hadron Collider (LHC) and includes an analysis of real UPC data collected by ATLAS experiment in 2025. This work focuses on rare processes with two photons in the final state, in particular light-by-light (LbyL) scattering.

Using the SuperChic and UPCgen MC generators, LbyL scattering was studied at the centre-of-mass energy of 5.36 TeV per nucleon pair, corresponding to the Run 3 conditions at the LHC. The main kinematic properties of this process were analysed. The MC cross section of the process within the ATLAS fiducial requirements was found to be 132.46 ± 0.54 nb. No significant dependence of the distribution shapes on the collision energy was observed. Different ion-breakup topologies were also analysed, obtaining a fraction of events with both nuclei remaining intact (0n0n) to all possible topologies (AnAn) of about 79%. Besides LbyL scattering, several other rare processes were investigated. In principle central exlusive production (CEP) gg → γγ including an evaluation of the uncertainties related to PDFs. For the first time, a detailed study was performed of dipion production (γγ → π⁰π⁰) using the UPCgen MC generator. The rare γγ → e⁺e⁻γγ process was also analysed using MadGraph5 MC generator. Both processes were shown to give a negligible contribution to the LbyL signal within the current ATLAS fiducial requirements.

An analysis of real UPC data from 2025 was performed to evaluate the efficiency of the vpix60 high-level trigger using γγ → e⁺e⁻ events selection. The trigger efficiency was found to be high and close to unity, although additional detector-related background was observed and will require further investigation.

Finally, feasibility studies of LbyL scattering analysis in the ALICE experiment phase space were carried out. In the ALICE fiducial acceptance, measurements at lower diphoton invariant masses are possible. The predicted signal yield (around 2300) and the controlled level of background indicate that a potential first LbyL scattering measurement in ALICE experiment is feasible.