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 4191-8+9 MeV/c2 and 65-16+22 MeV/c2, 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/c2. This contribution is larger than theoretical estimates.