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 rapidity 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. The horizontal error bars display the measurement bin width, 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 and without nuclear modification of the PDFs The filled blue boxes show the calculation using the CT14 PDF, while the green stippled boxes show the calculation using CT14 PDF with EPPS16 nPDF  All model calculations incorporate PDFs or nPDFs that account for the isospin of the Pb nucleus