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

TL;DR

This paper reports the first observation of a resonance in B+ to K+ mu+ mu- decays at low recoil, consistent with the psi(4160) meson, with measurements exceeding six sigma significance.

Contribution

It provides the first evidence of the decay B+ to psi(4160) K+ and the subsequent decay psi(4160) to mu+ mu-, with detailed resonance measurements.

Findings

Resonance consistent with psi(4160) meson

Resonance significance exceeds six standard deviations

Resonant decay accounts for 20% of low recoil yield

Abstract

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. 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 in the low recoil region, which is larger than theoretical estimates.