$Z$ boson production in Pb+Pb collisions at $\sqrt{s_{NN}}~=~5.02$ TeV measured by the ATLAS detector

The production of $Z$ bosons is measured in the dilepton, electrons and muons, decay channel in Pb+Pb collisions at $\sqrt{s_{NN}}~=~5.02$ TeV measured with the ATLAS detector. Data from the 2015 LHC run corresponding to an integrated luminosity of 0.49 $\textrm{nb}^{-1}$ are used for the analysis. $Z$ boson production is measured with respect to dilepton rapidity and event centrality. The measurements in Pb+Pb collisions are compared with similar measurements made in $pp$ collisions at the same center-of-mass energy. The nuclear modification factor, which quantifies the compatibility of the heavy ion and $pp$ collision results using the Glauber model calculation, is found to be consistent with unity in central and mid-central collisions. After rejecting the photon-induced background, the nuclear modification factor in peripheral collisions is above unity, although consistent with it. The nuclear modification factor measured as a function of rapidity agrees with unity while showing a trend consistent with a calculation including the isospin effect. The experimental data are compared with theoretical predictions obtained at the NLO using a proton and nuclear parton distribution functions. The measured $Z$ boson yields lie about 1-3$\sigma$ below the NLO predictions.