Observation of Light-by-Light Scattering in Ultraperipheral Pb +Pb Collisions with the ATLAS Detector

This Letter describes the observation of the light-by-light scattering process, γ γ →γ γ , in Pb +Pb collisions at √{s_NN }=5.02 TeV . The analysis is conducted using a data sample corresponding to an integrated luminosity of 1.73 nb^-1 , collected in November 2018 by the ATLAS experiment at the LHC. Light-by-light scattering candidates are selected in events with two photons produced exclusively, each with transverse energy E_T^γ>3 GeV and pseudorapidity |η_γ|<2.4 , diphoton invariant mass above 6 GeV, and small diphoton transverse momentum and acoplanarity. After applying all selection criteria, 59 candidate events are observed for a background expectation of 12 ±3 events. The observed excess of events over the expected background has a significance of 8.2 standard deviations. The measured fiducial cross section is 78 ±13 (stat) ±7 (syst) ±3 (lumi) nb .