Shedding Light on the Tau Lepton: Measuring photon-induced tau-lepton pairs to constrain the tau electromagnetic moments using √sNN = 5.02 TeV lead-lead collision data recorded by the ATLAS detector

Abstract

This thesis presents a novel measurement of photon-induced tau-lepton pair production in heavy-ion collisions which advances our understanding of the electromagnetic properties of the tau-lepton. It demonstrates the unique potential of ultra-peripheral lead-lead (Pb+Pb) collisions as a complementary probe of the Standard Model and beyond the Standard Model physics. The analysis uses Pb+Pb collision data recorded by the ATLAS detector at the Large Hadron Collider (LHC) in 2015 and 2018. Strong electromagnetic fields generated in ultra-peripheral Pb+Pb collisions give rise to substantial photon fluxes, providing a unique environment for studying photon-induced processes. Signal candidates are classified into three exclusive regions according to event topologies in which one tau-lepton decays into a muon and two neutrinos, while the decay of the second tau-lepton is characterised by the presence of either charged-particle track(s) or an electron. A strong suppression of ion breakup is ensured by requiring an absence of forward neutron activity. Differential cross sections are extracted using an Iterative Bayesian Unfolding approach. The results are compared with leading-order Standard Model predictions and found to be consistent within the measurement uncertainties, which are dominated by limited data statistics. The measurement enables extraction of the anomalous magnetic dipole moment g-2 and electric dipole moment (EDM) of the tau-lepton. Expected constraints are obtained through a statistical analysis and compared to the existing constraints. Performance studies of the ATLAS Forward Proton (AFP) detector are also presented. The AFP detector records protons scattered at small angles in photon-induced and diffractive processes, providing kinematic information that can be used to enhance background suppression. The global alignment of the AFP was performed using photon-induced dimuon events, recorded during LHC Run 3 proton-proton collisions.