Measurement of muon pairs produced via $\gamma \gamma$ 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 $\gamma \gamma \to {\mu }^{+}{\mu }^{-}$ 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.

arXiv comment75 pages in total, author list starting page 54, 25 figures, 3 tables, published in Physical Review C. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/HION-2020-10

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