Measurement of light-by-light scattering and search for axion-like particles with 2.2 nb$^{−1}$ of Pb+Pb data with the ATLAS detector

This note describes a measurement of light-by-light scattering based on Pb+Pb collision data recorded by the ATLAS experiment during the Run 2 of the LHC. The study is using 2.2 $\mathrm{nb^{-1}}$ of integrated luminosity collected in 2015 and 2018 at $\sqrt{s_\mathrm{NN}}=5.02$ TeV. Light-by-light scattering candidates are selected in events with two photons exclusively produced, each with transverse energy $E_{\textrm{T}}^{\gamma} > 2.5$ GeV, pseudorapidity $|\eta_{\gamma}| < 2.37$, diphoton invariant mass $m_{\gamma\gamma} > 5$ GeV, and with small diphoton transverse momentum and diphoton acoplanarity. The fiducial integrated and differential cross sections are measured and compared with theoretical predictions. The diphoton invariant mass distribution is used to set limits on the production of axion-like particles. This result provides the most stringent limits for diphoton masses of 6-100 GeV compared to previous results.