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−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−12.0 fb−1. Decay candidates with muon pairs that have an invariant mass in the range 675–875 MeV/c2675–875 MeV/c2 are considered. This region is dominated by the ρ0ρ0 and ω resonances. The branching fraction in this range is measured to beB(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−D0→K−π+μ+μ−. Its branching fraction is consistent with the value expected in the Standard Model.