Invariant yield of \dimuon from \zBoson in $2.5< y< 4.0$ divided by \taa (a) and nuclear modification factor (b) as a function of centrality (represented by \npartNcoll) for \PbPb collisions at $\snn=5.02$ TeV, considering muons with $-4.0< \etamu< -2.5$ and $\ptmu>20$\GeVc The vertical error bars are statistical only, while the boxes represent the systematic uncertainties The filled black box in panel (b), located at $\Raa=1$, shows the normalisation uncertainty The results are compared to theoretical calculations with centrality-dependent nPDFs that account for the isospin of the Pb nucleus