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

TL;DR

This paper reports the measurement of light-by-light scattering in lead-lead collisions at the LHC and sets new limits on axion-like particles in the 6-100 GeV mass range, using ATLAS data.

Contribution

First measurement of light-by-light scattering in Pb+Pb collisions at the LHC with ATLAS, and the most stringent limits to date on axion-like particles in the 6-100 GeV mass range.

Findings

Measured integrated and differential cross sections for light-by-light scattering.

Excluded axion-like particle production cross sections above 2 to 70 nb at 95% CL.

Set new limits on axion-like particles in the 6-100 GeV mass range.

Abstract

This paper describes a measurement of light-by-light scattering based on Pb+Pb collision data recorded by the ATLAS experiment during Run 2 of the LHC. The study uses $2.2$ 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 produced exclusively, each with transverse energy $E_{\mathrm{T}}^{\gamma} > 2.5$ GeV, pseudorapidity $|\eta_{\gamma}| < 2.37$, diphoton invariant mass $m_{\gamma\gamma} > 5$ GeV, and with small diphoton transverse momentum and diphoton acoplanarity. The integrated and differential fiducial 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 to date on axion-like particle production for masses in the range 6-100 GeV. Cross sections above 2 to 70 nb are excluded at the 95% CL in that mass interval.