  The production yield of $Z$ bosons is measured in the electron and muon decay
channels in Pb+Pb collisions at $\sqrt{s_{\textrm{NN}}}$ = 5.02 TeV 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. The $Z$ boson
yield, normalised by the total total number of minimum-bias events and the mean
nuclear thickness function, is measured as a function of dilepton rapidity and
event centrality. The measurements in Pb+Pb collisions are compared with
similar measurements made in proton$-$proton collisions at the same
centre-of-mass energy. The nuclear modification factor is found to be
consistent with unity for all centrality intervals. The results are compared
with theoretical predictions obtained at next-to-leading order using nucleon
and nuclear parton distribution functions. The normalised $Z$ boson yields in
Pb+Pb collisions lie 1$-3$$\sigma$ above the predictions. The nuclear
modification factor measured as a function of rapidity agrees with unity and is
consistent with a next-to-leading-order QCD calculation including the isospin
effect.

Nuclear Experiment;High Energy Physics - Experiment