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 Z + -(fwd) = 13.5 +5.4 -4.0 (stat.) 1.2(syst.) nb in the direction of the proton beam and Z + -(bwd) = 10.7 +8.4 -5.1 (stat.) 1.0(syst.) nb in the direction of the lead beam, where the first uncertainty is statistical and the second systematic.