Measurement of Z boson production cross-section in pp collisions at \sqrts = 5.02 TeV

The first measurement of the Z boson production cross-section at centre-of-mass energy √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 pT > 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 for the di-muon invariant mass in the range 60 < Mμμ < 120 GeV/c2. 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 pPb collisions at √sN N = 5.02 TeV, the nuclear modification fac- tor RpPb is measured for the first time at this energy. The measured val- ues are 1.2+0.5 −0.3 (stat) ± 0.1 (syst) in the forward region (1.53 < y∗ μ < 4.03) and 3.6+1.6 −0.9 (stat) ± 0.2 (syst) in the backward region (−4.97 < y∗ μ < −2.47), where y∗ μ represents the muon rapidity in the centre-of-mass frame