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

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Abstract

This document describes a measurement of the $Z\rightarrow \mu^+\mu^-$ 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\rightarrow \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\rightarrow \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.

Figures and captions

Control plots of the dimoun system after the offline selection, which includes the dimuon invariant mass ($m_{\mu ^+\mu ^- }$) distributions for $ p \mathrm{Pb}$ (a) and $\mathrm{Pb} p $ (b) configurations, the $ p_{\mathrm{T}}$ of the $ Z $ candidates ($ p_{\mathrm{T}} (Z)$) for $ p \mathrm{Pb}$ (c) and $\mathrm{Pb} p $ (d) configurations, and the rapidity of the dimuon system in the centre-of-mass frame ($y^*_Z$) for $ p \mathrm{Pb}$ (e) and $\mathrm{Pb} p $ (f) configurations. The red line shows the distributions from simulation generated using Pythia 8 with CTEQ6L1 PDF set, normalised to the number of observed candidates.

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(color online) Measured fiducial cross-sections of $ Z $ production and theoretical calculations using various PDF sets with or without nuclear modification.

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Comparison of LHCb 8.16 $\text{ Te V}$ results with previous 5.02 $\text{ Te V}$ results from ATLAS , CMS , ALICE , and LHCb . The uncertainties on the data over theory ratios include only the experimental statistical and systematic uncertainties; the PDF uncertainties are shown separately on the line at one. The central values of the LHCb and ALICE results at 5.02 $\text{ Te V}$ are shifted to left and right by 0.1 units in rapidity, respectively, for better visibility.

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Tables and captions

Summary of the relative uncertainties of the cross-section measurements for the $ p \mathrm{Pb}$ and $\mathrm{Pb} p $ configurations.

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Summary of the inputs to the fiducial cross-sections calculated in the common rapidity range $2.5<|y^*|<4.0$ for the forward-backward ratio $R_\mathrm{FB}$.

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Created on 06 March 2021.