First observation of the decay D0→K-π+μ+μ-in the ρ0-ω region of the dimuon mass spectrum

A study of the decay D0 → K −π+μ+μ− is performed using data collected by the LHCb detector in proton–proton collisions at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 2.0 fb−1. Decay candidates with muon pairs that have an invariant mass in the range 675–875 MeV/c2 are considered. This region is dominated by the ρ0 and ω resonances. The branching fraction in this range is measured to be B(D0 → K −π+μ+μ−) = (4.17 ± 0.12 (stat) ± 0.40 (syst)) × 10−6. This is the first observation of the decay D0 → K −π+μ+μ−. Its branching fraction is consistent with the value expected in the Standard Model.