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

Results are presented of a measurement of dimuons produced via $\gamma\gamma$ scattering processes in inelastic, non-ultra-peripheral Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV. The measurement uses ATLAS data from the 2015 and 2018 Pb+Pb data-taking periods at the LHC with an integrated luminosity of $1.9~\mathrm{nb}^{-1}$. The $\gamma\gamma \rightarrow \mu^+\mu^-$ pairs are identified via selections on pair momentum asymmetry and acoplanarity, and the contribution from the heavy flavor decay background is estimated using a template fit method. The pair yields are measured differentially as functions of the centrality, average transverse-momentum ($\bar{p}_{\mathrm{T}}$) and rapidity of the pair. The acoplanarity distribution in a given centrality interval is observed to vary with $\bar{p}_{\mathrm{T}}$. In contrast, distributions of the perpendicular transverse momentum, $k_\perp \equiv \bar{p}_{\mathrm{T}} |\pi-\Delta \phi|$, are nearly independent of the $\bar{p}_{\mathrm{T}}$. The prior observation of a collision centrality-dependent broadening of the acoplanarity distribution is confirmed. Furthermore, the improved statistics of the measurement reveal a depletion in the number of muon pairs near zero acoplanarity or $k_{\perp}$ in central events, resulting in the distributions peaking at non-zero values of acoplanarity or $k_{\perp}$. Fits to the $k_\perp$ distributions are used to estimate the centrality dependence of the peak position. The most probable $k_\perp$ is shown to increase from the most peripheral to the most central collisions, reaching a value of $k_{\perp}=36\pm1$ MeV in the 0--5% most-central collisions. The ability of these measurements to qualitatively differentiate between different physical origins of the observed centrality and $\bar{p}_{\mathrm{T}}$ dependence are discussed.