Measurement of muon pairs produced via γγ scattering in nonultraperipheral Pb + Pb collisions at SQUARE ROOTsNN = 5.02 TeV with the ATLAS detector
Abstract
Results of a measurement of dimuon photoproduction in nonultraperipheral Pb + Pb collisions at SQUARE ROOTsNN = 5.02 TeV are presented. Themeasurement uses ATLAS data from the 2015 and 2018 Pb + Pb data-taking periods at the LHC with an integrated luminosity of 1.94 nb-1. The γγ RIGHTWARDS ARROW μ
+
μ
- 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 kUP TACK , 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 kUP TACK values in more central collisions. The acoplanarity distributions in a given centrality interval are observed to vary with the mean pT of the muons in the pair, but the kUP TACK 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.