Invariant yield of \dimuon from \zBoson production in $2.5< y< 4.0$ divided by the average nuclear overlap function in the \centr{0}{90} centrality class, considering muons with $-4.0< \etamu< -2.5$ and $\ptmu>20$\GeVc. The horizontal solid line represents the statistical uncertainty of the measurement while the yellow filled band shows the systematic uncertainty. The result is compared to theoretical calculations with and without nuclear modification of the PDFs~ All model calculations incorporate PDFs or nPDFs determined by considering the isospin of the Pb-nucleus |