Observation of centrality-dependent acoplanarity for muon pairs produced via two-photon scattering in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with the ATLAS detector

TL;DR

This study measures muon pair production via two-photon scattering in lead-lead collisions at the LHC, revealing that muon angular correlations broaden with increased collision centrality, indicating interactions with hot nuclear matter.

Contribution

First measurement of centrality-dependent muon pair acoplanarity in Pb+Pb collisions at 5.02 TeV using the ATLAS detector, highlighting medium effects on electromagnetic processes.

Findings

Muon pairs show increased acoplanarity in central collisions.

Angular correlations broaden significantly with collision centrality.

Results are consistent with rescattering of muons in hot nuclear matter.

Abstract

This Letter presents a measurement of $\gamma \gamma \rightarrow \mu^{+} \mu^{-}$ production in Pb+Pb collisions recorded by the ATLAS detector at the Large Hadron Collider at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with an integrated luminosity of 0.49 nb$^{-1}$. The azimuthal angle and transverse momentum correlations between the muons are measured as a function of collision centrality. The muon pairs are produced from $\gamma \gamma$ through the interaction of the large electromagnetic fields of the nuclei. The contribution from background sources of muon pairs is removed using a template fit method. In peripheral collisions, the muons exhibit a strong back-to-back correlation consistent with previous measurements of muon pair production in ultra-peripheral collisions. The angular correlations are observed to broaden significantly in central collisions. The modifications are qualitatively consistent with rescattering of the muons while passing through the hot matter produced in the collision.