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 ${4191}_{-8}^{+9}\mathrm{MeV}/c^2$ and ${65}_{-16}^{+22}\mathrm{MeV}/c^2$, 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 for dimuon masses above $3770\mathrm{MeV}/c^2$. This contribution is larger than theoretical estimates.

• Received 29 July 2013

DOI:https://doi.org/10.1103/PhysRevLett.111.112003

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© 2013 CERN, for the LHCb collaboration