First observation of the decay $D^0\to K^{-}{\pi }^{+}{\mu }^{+}{\mu }^{-}$ in the ${\rho }^0$-$\omega$ region of the dimuon mass spectrum

A study of the decay $D^0\to K^{-}{\pi }^{+}{\mu }^{+}{\mu }^{-}$ 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$\mathrm{/}{c}^2$ are considered. This region is dominated by the ${\rho }^0$ and $\omega$ resonances. The branching fraction in this range is measured to be $\mathcal{B}$($D^0\to K^{-}{\pi }^{+}{\mu }^{+}{\mu }^{-}$) = $(4.17\pm 0.12(stat)\pm 0.40(syst))\times {10}^{-6}$. This is the first observation of the decay $D^0\to K^{-}{\pi }^{+}{\mu }^{+}{\mu }^{-}$. Its branching fraction is consistent with the value expected in the Standard Model.