Observation of a Resonance in B+→K+μ+μ− Decays at Low Recoil

A broad peaking structure is observed in the dimuon spectrum of B + → K + μ + μ − 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 + → K + μ + μ − 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 MeV / c 2 and 65 + 22 − 16 MeV / c 2 , respectively, where the uncertainties include statistical and systematic contributions. These measurements are compatible with the properties of the ψ ( 4160 ) meson. First observations of both the decay B + → ψ ( 4160 ) K + and the subsequent decay ψ ( 4160 ) → μ + μ − are reported. The resonant decay and the interference contribution make up 20% of the yield for dimuon masses above 3770 MeV / c 2 . This contribution is larger than theoretical estimates