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

Understand

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 $4191^{+9}_{-8}\mathrm{\,Me\kern -0.1em V/}c^2$ 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 $\psi(4160)$ meson.