Measurement of Z -boson production at large rapidities in Pb–Pb collisions at

The production of ${\text{Z}}^{0}$ bosons at large rapidities in Pb–Pb collisions at $\sqrt{{s}_{\mathrm{NN}}}=5.02\phantom{\rule{0.25em}{0ex}}\text{TeV}$ is reported. ${\text{Z}}^{0}$ candidates are reconstructed in the dimuon decay channel (${\text{Z}}^{0}\to {\mu }^{+}\phantom{\rule{0.2em}{0ex}}{\mu }^{-}$), based on muons selected with pseudo-rapidity $-4.0<\eta <-2.5$ and ${p}_{\text{T}}>20\phantom{\rule{0.25em}{0ex}}\text{GeV/}c$. The invariant yield and the nuclear modification factor, ${R}_{\text{AA}}$, are presented as a function of rapidity and collision centrality. The value of ${R}_{\text{AA}}$ for the 0–20% central Pb–Pb collisions is $0.67±0.11\phantom{\rule{0.2em}{0ex}}\text{(stat.)}\phantom{\rule{0.2em}{0ex}}±0.03\phantom{\rule{0.2em}{0ex}}\text{(syst.)}\phantom{\rule{0.2em}{0ex}}±0.06\phantom{\rule{0.2em}{0ex}}\text{(corr. syst.)}$, exhibiting a deviation of 2.6σ from unity. The results are well-described by calculations that include nuclear modifications of the parton distribution functions, while the predictions using vacuum PDFs deviate from data by 2.3σ in the 0–90% centrality class and by 3σ in the 0–20% central collisions.