Ultra-Peripheral Collisions physics with ATLAS

Exclusive production of dilepton pairs, $\gamma\gamma \to \ell\ell, \ell = e, \mu, \tau$, is studied using data from ultraperipheral collisions (UPC) of lead nuclei at $\sqrt{s_{NN}}$=5.02~TeV recorded by the ATLAS detector~\cite{atlas} at the LHC. The process of interest proceeds via photon--photon interactions in the strong electromagnetic fields of relativistic lead nuclei. In the case of the dielectron production, within experimental uncertainties the measured integrated cross section is in good agreement with the QED predictions from the Monte Carlo programs STARlight and SuperChic, confirming the broad features of the initial photon fluxes. The differential cross section shows systematic differences from these predictions which are more pronounced at high absolute rapidity of the $ee$ system $|y_{ee}|$ and $|\cos{\theta}^{*}|$ values. In the dimuon production case, the measured cross sections at large $|y_{\mu \mu}|$ is found to be about 10-20\% larger in data than in the calculations, suggesting the presence of larger fluxes of photons in the initial state. The $\gamma\gamma\to \tau\tau$ process is observed in the UPCs collisions with a significance exceeding 5 standard deviations. This observation can be turned into a constraint on the $\tau$-lepton anomalous magnetic moment. Measurement of light-by-light scattering data are also performed during the LHC Run~2. The diphoton invariant mass distribution is used to set limits on the production of axion-like particles. This result provides the most stringent limits to date on axion-like particle production for masses in the range 6-100~GeV.