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

A bstract The first measurement of the Z boson production cross-section at centre-of-mass energy $$ \sqrt{s} $$ 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 $$ {\sigma}_{Z\to \mu +\mu -}=39.6\pm 0.7\left(\textrm{stat}\right)\pm 0.6\left(\textrm{syst}\right)\pm 0.8\left(\textrm{lumi}\right)\textrm{pb} $$ 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 $$ \sqrt{s_{NN}} $$ 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 < $$ {y}_{\mu}^{\ast } $$ 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 < $$ {y}_{\mu}^{\ast } $$ y μ ∗ < − 2 . 47), where $$ {y}_{\mu}^{\ast } $$ y μ ∗ represents the muon rapidity in the centre-of-mass frame.