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

A broad peaking structure is observed in the dimuon spectrum of $B^+ \to K^+ μ^+μ^-$ decays in the kinematic region where the kaon has a low recoil against the dimuon system. The structure is consistent with interference between the $B^+ \to K^+ μ^+μ^-$ decay and a resonance and has a statistical significance exceeding six standard deviations. The mean and width of the resonance are measured to be $4191^{+9}_{-8}\mathrm{\,Me\kern -0.1em V/}c^2$ and $65^{+22}_{-16}\mathrm{\,Me\kern -0.1em V/}c^2$, 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 $B^+ \to ψ(4160) K^+$ and the subsequent decay $ψ(4160) \to μ^+μ^-$ 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.