Observation of a resonance in $B^{+}\to K^{+}{\mu }^{+}{\mu }^{-}$ decays at low recoil

A broad peaking structure is observed in the dimuon spectrum of $B^+ \to K^+ \mu^+\mu^-$ decays in the kinematic region where the kaon has a low recoil against the dimuon system. The structure is consistent with interference between the $B^{+}\to K^{+}{\mu }^{+}{\mu }^{-}$ decay and a resonance and has a statistical significance exceeding six standard deviations. The mean and width of the resonance are measured to be $4191^{+9}{-8}\mathrm{,Me\kern -0.1em V/}c^2$and$65^{+22}{-16}$\mathrm{M}\mathrm{e}\mathrm{V}\mathrm{/}$c^2$, respectively, where the uncertainties include statistical and systematic contributions. These measurements are compatible with the properties of the $\psi (4160)$ meson. First observations of both the decay $B^{+}\to \psi (4160)K^{+}$ and the subsequent decay $\psi (4160)\to {\mu }^{+}{\mu }^{-}$ are reported. The resonant decay and the interference contribution make up 20,% of the yield in the low recoil region, which is larger than theoretical estimates.