Observation of Associated Production of a Z Boson with a D Meson in the Forward Region

A search for associated production of a Z boson with an open charm meson is presented using a data sample, corresponding to an integrated luminosity of 1.0 fb^−1 of proton-proton collisions at a centre-of-mass energy of 7 TeV, collected by the LHCb experiment. Seven candidate events for associated production of a Z boson with a D0 meson and four candidate events for a Z boson with a D+ meson are observed with a combined significance of 5.1 standard deviations. The production cross-sections in the forward region are measured to be σ(Z → μ+ μ−, D0) = 2.50 ± 1.12 ± 0.22 pb, σ(Z → μ+ μ−, D+) = 0.44 ± 0.23 ± 0.03 pb, where the first uncertainty is statistical and the second systematic.