Observation of light-by-light scattering in ultraperipheral Pb+Pb collisions with the ATLAS detector

This note describes the observation of the light-by-light scattering process, $\gamma\gamma\to\gamma\gamma$, in Pb+Pb collisions at $\sqrt{s_\mathrm{NN}}$=5.02 TeV. The analysis is conducted using a data sample corresponding to an integrated luminosity of 1.73 $\mathrm{nb^{-1}}$, 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 $E_{\textrm{T}}^{\gamma} > 3\; GeV$ and pseudorapidity $|\eta_{\gamma}| < 2.37$, and with diphoton invariant mass $m_{\gamma\gamma} > 6\;GeV$, and with 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. An excess of events over the expected background is found with an observed significance of 8.2 standard deviations. The measured fiducial cross section is $78 \pm 13 (\mathrm{stat.}) \pm 7 (\mathrm{syst.}) \pm 3 (\mathrm{lumi.})$ nb.