Measurement of muon pairs produced via $γγ$ scattering in non-ultraperipheral Pb+Pb collisions at $\sqrt{s_{_\mathrm{NN}}} = 5.02$ TeV with the ATLAS detector

Abstract: Results of a measurement of dimuon photoproduction in non-ultraperipheral Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV are presented. The measurement uses ATLAS data from the 2015 and 2018 Pb+Pb data-taking periods at the LHC with an integrated luminosity of $1.94~\mathrm{nb}^{-1}$. The $\gamma\gamma \rightarrow \mu^+\mu^-$ pairs are identified via selections on pair momentum asymmetry and acoplanarity. Differential cross-sections for dimuon production are measured in different centrality, average muon momentum and pair rapidity intervals as functions of acoplanarity and $k\perp$, the transverse momentum kick of one muon relative to the other. Measurements are also made as a function of the rapidity separation of the muons and the angle of the muon pair relative to the second-order event plane to test whether magnetic fields generated in the quark-gluon plasma affect the measured muons. A prior observation of a centrality-dependent broadening of the acoplanarity distribution is confirmed. Furthermore, the improved precision of the measurement reveals a depletion in the number of pairs having small acoplanarity or $k_{\perp}$ values in more central collisions. The acoplanarity distributions in a given centrality interval are observed to vary with the mean-$p_{\mathrm{T}}$ of the muons in the pair, but the $k_{\perp}$ distributions do not. Comparisons with recent theoretical predictions are made. The predicted trends associated with effects of magnetic fields on the dimuons are not observed.