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

## TL;DR

This paper reports the first observation of light-by-light scattering in ultraperipheral lead-lead collisions at the LHC, with a significant excess of events over background and a measured cross section consistent with theoretical predictions.

## Contribution

It provides the first experimental evidence of light-by-light scattering in heavy-ion collisions using the ATLAS detector.

## Key findings

- Observation of 59 candidate events with 8.2 sigma significance
- Measured fiducial cross section of 78 ± 13 (stat.) ± 7 (syst.) ± 3 (lumi.) nb
- Background expectation of 12 ± 3 events

## Abstract

This letter describes the observation of the light-by-light scattering process, $\gamma\gamma\rightarrow\gamma\gamma$, 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 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 $|\eta_{\gamma}| < 2.37$, 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.