Figure 1

 Invariant mass distribution of \dimuon pairs for \PbPb collisions at $\snn=5.02$~TeV, reconstructed using muons with $-4.0< \etamu< -2.5$ and $\ptmu>20$\GeVc (black points) The panels present the distribution in different centrality intervals. The error bars are of statistical origin only The invariant mass distribution of like-sign muon pairs is also shown (red open points). Only one like-sign candidate is found in the \centr{20}{90} centrality interval The solid blue line drawn in \fig{fig:ZRawYield_0_90} represents the distribution from a POWHEG simulation for \PbPb collisions without nuclear modification of PDFs (see text for details)