Study of J/ψ polarization in Pb-Pb collisions at √SNN = 5.02 TeV with the ATLAS experiment

Abstract

The polarization of the J/ψ is studied with respect to the axis perpendicular to the event plane in the laboratory frame, using J/ψ mesons produced in Pb-Pb collisions at √sNN = 5.02 TeV. The analysis is based on data collected by the ATLAS detector during the 2023 and 2024 heavy-ion runs. The measurement is performed by analyzing the angular distribution of the J/ψ decay in the dimuon channel, in the rapidity region |y| < 2.0, for several transverse momentum intervals in the range 8 < pT < 24 GeV and for different centrality classes. The objective is to identify kinematic and centrality regions where there is strong evidence of polarization and to determine where sufficient statistical precision exists to further investigate this effect. The results are reported for regions in which hypothesis testing indicates the presence of polarization, revealing a dependence on transverse momentum and collision centrality. Owing to the chosen reference frame, the observed polarization may be associated with the properties and dynamics of the quark-gluon plasma formed in Pb-Pb collisions.