Measurement of Z⁰-boson production at large rapidities in Pb–Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\text{TeV}$

Abstract

The production of ${\text{Z}}^0$ bosons at large rapidities in Pb–Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\text{TeV}$ is reported. ${\text{Z}}^0$ candidates are reconstructed in the dimuon decay channel (${\text{Z}}^0\to {\mu }^{+}{\mu }^{-}$), based on muons selected with pseudo-rapidity $-4.0<\eta <-2.5$ and $p_{\text{T}}>20\text{GeV/}c$. 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 $0.67\pm 0.11\text{(stat.)}\pm 0.03\text{(syst.)}\pm 0.06\text{(corr. syst.)}$, 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.