Observation of Light-by-Light Scattering in Ultraperipheral $\mathrm{Pb}+\mathrm{Pb}$ Collisions with the ATLAS Detector

This Letter describes the observation of the light-by-light scattering process, $\gamma \gamma \to \gamma \gamma $, in $\mathrm{Pb}+\mathrm{Pb}$ collisions at $\sqrt{{s}_{\mathrm{NN}}}=5.02\text{}\text{}\mathrm{TeV}$. The analysis is conducted using a data sample corresponding to an integrated luminosity of $1.73\text{}\text{}{\mathrm{nb}}^{-1}$, 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 ${E}_{T}^{\gamma }>3\text{}\text{}\mathrm{GeV}$ and pseudorapidity $|{\eta }_{\gamma }|<2.4$, 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 $78±13\left(\text{stat}\right)±7\left(\text{syst}\right)±3\left(\text{lumi}\right)\text{}\text{}\mathrm{nb}$.