Light-by-light scattering in the ATLAS detector at the LHC

This memorandum reports the study of the light-by-light scattering process, γγ → γγ, in Pb+Pb collisions at the ATLAS experiment. The study uses 2.2 nb−1 of integrated luminosity collected in 2015 and 2018 at √sN N = 5.02 T e V . Light-by-light scattering candidates are selected in events with two photons produced exclusively, each with transverse energy ETγ > 2.5 GeV and pseudorapidity |η| < 2.37, diphoton invariant mass above 5 GeV , and small diphoton transverse momentum and acoplanarity. After applying all selection criteria, 97 candidate events are observed for a background expectation of 27 ± 5 events. The integrated and differential cross sections are measured and compared with theoretical predictions. The measured fiducial cross section is 121.53 ± 17.03 (stat.) ± 9.51 (syst.) ± 3.83 (lumi.) nb. The observed excess of events over the expected background has a significance of 12.45 standard deviations.