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, $\gamma \gamma \to \gamma \gamma$, in Pb+Pb collisions at $\sqrt{s_{\mathrm{N}\mathrm{N}}}$ = 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 $E_{\text{T}}^{\gamma }>3$ GeV and pseudorapidity $\mathrm{\mid }{\eta }_{\gamma }\mathrm{\mid }<2.37$, 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 $\pm$ 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 $\pm$ 13 (stat.) $\pm$ 7 (syst.) $\pm$ 3 (lumi.) nb.