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

Abstract

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 $4191_{-8}^{+9}\mathrm{Me}V/c^2$ and $65_{-16}^{+22}\mathrm{Me}V/c^2$, 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.

8 pages, 3 figures, updated to correspond to proofs from PRL

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