Light-by-light Scattering in Lead--Lead Collisions in the ATLAS Experiment

Light-by-light (LbyL) scattering, ! , is a quantum-mechanical process, forbidden by the classical theory of electrodynamics, but possible in Quantum Electrodynamics via a loop diagram. Despite the small cross section, it is theoretically possible to observe this process in ultra-peripheral high-energy heavy-ion collisions. Based on 0.48 nb􀀀1 of 2015 Pb+Pb data, a first direct evidence of LbyL scattering was established by the ATLAS Collaboration in 2017. In total, 13 events were found in the signal region with a background expectation of 2:60:7 events. The excess corresponds to 4.4 significance over the background-only hypothesis. In November 2018, the new dataset of Pb+Pb collisions was collected by the ATLAS experiment with an integrated online luminosity of 1.7 nb􀀀1. This recent dataset has been employed to perform a preliminary study using the control sample from ! e+e􀀀 process.