Illuminating the tau lepton with the ATLAS detector: A study of $\gamma\gamma\rightarrow\tau^+\tau^-$ scattering in ultra-peripheral Pb+Pb collisions, and constraints on the tau lepton electromagnetic dipole moments

Lead ions accelerated to ultra-relativistic energies at the Large Hadron Collider are accompanied by a strong flux of quasi-real photons. These photons can interact and produce pairs of tau leptons, via the $\gamma \gamma \to \tau ^{+}\tau ^{-}$ scattering process, at an enhanced rate in ultra-peripheral Pb+Pb collisions. This process can be used to investigate the physics of the photon-tau lepton coupling. This thesis presents the first measurement of the $ \gamma \gamma \to \tau ^{+}\tau ^{-}$ scattering cross-section in ultra-peripheral Pb+Pb collisions at $\sqrt{^{s}NN}$ = 5.02 TeV, recorded by the ATLAS detector in Run 2 of the Large Hadron Collider. The cross-sections are measured for 21 kinematic observables of the tau lepton decay products across three fiducial regions, and are found to agree well with the available theoretical predictions. The measured cross-sections are used to constrain the anomalous magnetic dipole moment and the electric dipole moment of the tau lepton. The obtained constraints are expected to be the most stringent results using $\sqrt{^{s}NN}$ = 5.02 TeV Pb+Pb collisions at the Large Hadron Collider.