Search for the rare decay $B^{+} \rightarrow \mu^{+}\mu^{-}\mu^{+}\nu_{\mu}$

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A search for the rare leptonic decay $B^{+} \rightarrow {\mu}^{+}{\mu}^{-}{\mu}^{+}{\nu}_{{\mu}}$ is performed using proton-proton collision data corresponding to an integrated luminosity of $4.7$ fb$^{-1}$ collected by the LHCb experiment. The search is carried out in the region where the lowest of the two ${\mu}^{+}{\mu}^{-}$ mass combinations is below $980$MeV/c$^{2}$. The data are consistent with the background-only hypothesis and an upper limit of $1.6 \times 10^{-8}$ at 95 confidence level is set on the branching fraction in the stated kinematic region.

Figures and captions

Feynman diagrams of the contributions (top left) $ B ^+ \rightarrow \mu ^+ \nu _\mu \gamma^{*}$ with $\gamma^{*}\rightarrow \mu ^+\mu ^- $, (top right) $ B ^+ \rightarrow \mu ^+ \nu _\mu V$ and (bottom) bremsstrahlung to the $ B ^+ \rightarrow \mu ^+ \mu ^- \mu ^+ \nu _\mu $ decay.

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Figure_1B.eps [169 KiB]
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Fit to the mass distribution of the selected $ B ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^+ $ candidates. The combinatorial background (purple) and misidentified $ B ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} \pi ^+ $ decays (orange) are stacked up while the $ B ^+ \rightarrow { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} K ^+ $ signal is shown as a dashed line. The data points are shown as black points with the total fit overlaid as a red solid line.

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Template distributions for signal and misidentified background shapes for high and low fractional corrected mass uncertainty. A low uncertainty on the corrected mass corresponds to data with better mass resolution. The shape of the misidentification template is obtained from a control sample while the signal template is obtained from simulation. A systematic uncertainty on the signal shape due to the choice of the signal model is not shown, as it is too small to be visible.

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Corrected mass distribution of the selected $ B ^+ \rightarrow \mu ^+ \mu ^- \mu ^+ \nu _\mu $ candidates with the fit overlaid. Samples with low and high corrected mass uncertainty are fitted as individual samples but are merged in the figure. The fit has components for (green) combinatorial background, (blue) misidentified candidates and (orange) partially reconstructed candidates. The signal component is not visible as the fitted signal yield is negative. The best fit is the solid red line while the dashed line shows how the total would have looked like if the signal had the branching fraction predicted in Ref. [11].

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Animated gif made out of all figures.


Tables and captions

Summary of systematic uncertainties. Numbers are on the relative uncertainty of the normalisation for the branching fraction of the signal.

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Created on 06 March 2021.