Measurement of $Z$ production cross-sections in proton-lead collisions at $ \sqrt{s_\rm{NN}}=8.16\ \rm{TeV}$

This document describes a measurement of the $Z\to\mu^+\mu^-$ production cross-sections in proton-lead collisions at a centre-of-mass energy per nucleon pair of $\sqrt{s_\mathrm{NN}}=8.16\ \mathrm{TeV}$, using data collected by the LHCb detector in 2016. The dataset corresponds to an integrated luminosity of $30.8\,\mathrm{nb}^{-1}$, with $12.2\,\mathrm{nb}^{-1}$ for the forward ($p$Pb) configuration and $18.6\,\mathrm{nb}^{-1}$ for the backward (Pb$p$) configuration. The $Z$ candidates are reconstructed from two opposite-sign muons with transverse momentum greater than $20\,\mathrm{GeV}$, pseudorapidity from 2.0 to 4.5, and an invariant dimuon mass between 60 and 120 GeV. The measured cross-section in the LHCb acceptance is \begin{equation*} \sigma_{Z\to\mu^+\mu^-,\:p\mathrm{Pb}} = ~28.5 \pm 1.7(\mathrm{stat}) \pm 1.2(\mathrm{syst}) \pm 0.7(\mathrm{lumi}) \mathrm{nb}, \end{equation*} in the case when the proton propagates in the positive z-direction and \begin{equation*} \sigma_{Z\to\mu^+\mu^-,\:\mathrm{Pb}p} = ~13.4 \pm 1.0(\mathrm{stat}) \pm 1.4(\mathrm{syst}) \pm 0.3(\mathrm{lumi}) \mathrm{nb}, \end{equation*} in the case when it is traveling in the opposite direction.