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

The frst 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 fnal-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/c 2 . This result and the diferential 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 pPb collisions at √ sNN = 5.02 TeV, the nuclear modifcation factor RpPb is measured for the frst time at this energy. The measured values 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.