Measurement of the Z boson production cross-section in pp collisions at √s = 5.02 TeV

A bstract The first measurement of the Z boson production cross-section at centre-of-mass energy √s 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 ± 2 pb − 1 . The production cross-section is measured for final-state muons in the pseudorapidity range 2 . 0 < η < 4 . 5 with transverse momentum p T > 20 GeV/ c . The integrated cross-section is determined to be σZ→ μ +μ -=39.6± 0.7(stat)± 0.6(syst)± 0.8(lumi)pb σ Z → μ + μ − = 39.6 ± 0.7 stat ± 0.6 syst ± 0.8 lumi pb for the di-muon invariant mass in the range 60 < M μμ < 120 GeV/ 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 constant. Based on a previous LHCb measurement of the Z boson production cross-section in p Pb collisions at √sNN s NN = 5 . 02 TeV, the nuclear modification factor R p Pb is measured for the first time at this energy. The measured values are 1.2-0.3+0.5 1.2 − 0.3 + 0.5 (stat) ± 0 . 1(syst) in the forward region (1.53 < yμ y μ ∗ < 4 . 03) and 3.6-0.9+1.6 3.6 − 0.9 + 1.6 (stat) ± 0 . 2(syst) in the backward region ( − 4 . 97 < yμ y μ ∗ < − 2 . 47), where yμ y μ ∗ represents the muon rapidity in the centre-of-mass frame.