Observation of Z production in proton-lead collisions at LHCb

The first observation of Z boson production in proton-lead collisions at a centre-of-mass energy per proton-nucleon pair of s N N = 5 TeV is presented. The data sample corresponds to an integrated luminosity of 1 . 6 nb −1 collected with the LHCb detector. The Z candidates are reconstructed from pairs of oppositely charged muons with pseudorapidities between 2.0 and 4.5 and transverse momenta above 20 GeV / c. The invariant dimuon mass is restricted to the range 60 − 120 GeV / c. The Z production cross-section is measured to be σ Z → μ + μ − fwd = 13.5 − 4.0 + 5.4 stat . ± 1.2 syst . nb