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Search for the lepton flavour violating decay $B^+ \rightarrow K^+ \mu^- \tau^+$ using $B_{s2}^{*0}$ decays

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Abstract

A search is presented for the lepton flavour violating decay $B^+ \rightarrow K^+ \mu^- \tau^+$ using a sample of proton--proton collisions at centre-of-mass energies of 7, 8, and 13 TeV, collected with the LHCb detector and corresponding to a total integrated luminosity of 9 fb${}^{-1}$. The $\tau$ leptons are selected inclusively, primarily via decays with a single charged particle. The four-momentum of the $\tau$ lepton is determined by using $B^+$ mesons from $B_{s2}^{*0} \rightarrow B^+ K^-$ decays. No significant excess is observed, and an upper limit is set on the branching fraction $B(B^+ \rightarrow K^+ \mu^- \tau^+) < 3.9 \times 10^{-5}$ at 90 confidence level. The obtained limit is comparable to the world-best limit.

Figures and captions

Missing mass squared, $ m_{\mathrm{miss}}^2$ , distributions for (left) simulated signal $ B ^+ \rightarrow K ^+ \mu ^- \tau ^+ $ decays and (right) all selected candidates in data before applying the signal optimisation described in \cref{sec:signal}.

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Distributions of normalization candidates in (left) mass, $m_{ K ^+ \mu ^- \mu ^+ }$, and (right) the mass difference, $m_{ B ^+ K ^- } - m_{ B ^+ } - m_{ K ^- }$. The result of each fit is shown as a solid line, with the background component as a dashed line.

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Fits to the missing-mass-squared distribution \text{OS} $ K $ signal sample in each bin of BDT output included in the final fit. The best fit is overlaid. BDT bin 1 is the most background-like. The fit is performed using a fifth degree polynomial description of the background.

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Scan of the $p$-value in the signal branching fraction used to determine the CLs upper limits, compared to the expected one. The horizontal red line shows a $p$-value of 0.1, used to define the 90% CL upper limit.

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Animated gif made out of all figures.

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Created on 27 February 2021.