Measurement of the Z boson production cross-section in pp collisions at s = 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 p _T> 20 GeV/c. The integrated cross-section is determined to be (Formula presented.) 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 pPb collisions at s _NN = 5.02 TeV, the nuclear modification factor R _pPb is measured for the first time at this energy. The measured values are 1.2 _−0.3 ^+0.5 (stat) ± 0.1(syst) in the forward region (1.53 <y _μ ^∗< 4.03) and 3.6 _−0.9 ^+1.6 (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. (Figure presented.)