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

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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 $4191^{+9}_{-8}\mathrm{ Me V/}c^2$ and $65^{+22}_{-16}\mathrm{ Me V/}c^2$, 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.

Figures and captions

Dimuon mass distribution of data with fit results overlaid for the fit that includes contributions from the non-resonant vector and axial vector components, and the $\psi (3770)$ , $\psi(4040)$, and $\psi(4160)$ resonances. Interference terms are included and the relative strong phases are left free in the fit.

q2fit_[..].pdf [37 KiB]
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q2fit_with4140_and4040.pdf

Profile likelihood ratios for the product of branching fractions $\cal B ( B ^+ \rightarrow \psi K ^+ )\times\cal B (\psi \rightarrow \mu ^+\mu ^- )$ of the $\psi(4040)$ and the $\psi(4160)$ mesons. At each point all other fit parameters are reoptimised.

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4160_likelihood4040_likelihood.pdf

Profile likelihood as a function of mass and width of a fit with a single extra resonance. At each point all other fit parameters are reoptimised. The three ellipses are (red-solid) the best fit and previous measurements of (grey-dashed) the $\psi(4160)$ [4] and (black-dotted) the $ Y (4260)$ [22] states.

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Animated gif made out of all figures.

PAPER-2013-039.gif
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Tables and captions

Parameters of the dominant resonance for fits where the mass and width are unconstrained and constrained to those of the $\psi(4160)$ meson [4], respectively. The branching fractions are for the $ B ^+ $ decay followed by the decay of the resonance to muons.

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Table_1.pdf

Supplementary Material [file]

Supplementary material full pdf

supple[..].pdf [367 KiB]
supplementary.pdf

This ZIP file contains supplementary material for the publication LHCb-PAPER-2013-039. The files are: Supplementary.pdf: An overview of the extra figure *.pdf, *.png, *.eps, *.C: The figures in various formats. The *.C files contain the data on the figures.

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Created on 20 February 2021.