Measurement of the $Z$ boson production cross-section in $pp$ collisions at $\sqrt{s}=5.02$ TeV

Abstract

The first measurement of the $Z$ boson production cross-section at centre-of-mass energy $\sqrt{s}=5.02$TeV in the forward region is reported, using $pp$ collision data collected by the LHCb experiment in year 2017, corresponding to an integrated luminosity of $100\pm 2\mathrm{p}{\mathrm{b}}^{-1}$. The production cross-section is measured for final-state muons in the pseudorapidity range $2.0<\hat{I}\cdot <4.5$ with transverse momentum $p_{\mathrm{T}}>20GeV/c$. The integrated cross-section is determined to be \[ σ_{Z \rightarrow μ^{+}μ^{-}} = 39.6 \pm 0.7\,(\rm{stat}) \pm 0.6\,(\rm{syst}) \pm 0.8\,(\rm{lumi}) \ \rm{pb} \] for the di-muon invariant mass in the range $60<M_{\hat{I}¼\hat{I}¼}<120GeV/c^2$. This result and the differential cross-section results are in good agreement with theoretical predictions at next-to-next-to-leading order in the strong coupling. Based on a previous LHCb measurement of the $Z$ boson production cross-section in $p$Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV, the nuclear modification factor $R_{p\mathrm{Pb}}$ is measured for the first time at this energy. The measured values are $1.2_{-0.3}^{+0.5}(\mathrm{stat})\pm 0.1(\mathrm{syst})$ in the forward region ($1.53<y_{\hat{I}}^{\ast }¼<4.03$) and $3.6_{-0.9}^{+1.6}(\mathrm{stat})\pm 0.2(\mathrm{syst})$ in the backward region ($-4.97<y_{\hat{I}}^{\ast }¼<-2.47$), where $y_{\hat{I}}^{\ast }¼$ represents the muon rapidity in the centre-of-mass frame.

All figures and tables, along with any supplementary material and additional information, are available at https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2023-010.html (LHCb public pages)