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Search for the lepton flavour violating decay $\tau^-\to \mu^-\mu^+\mu^-$

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Abstract

A search for the lepton flavour violating decay $\tau^-\to \mu^-\mu^+\mu^-$ is performed with the LHCb experiment. The data sample corresponds to an integrated luminosity of $1.0\mathrm{\,fb}^{-1}$ of proton-proton collisions at a centre-of-mass energy of $7\mathrm{\,TeV}$ and $2.0\mathrm{\,fb}^{-1}$ at $8\mathrm{\,TeV}$. No evidence is found for a signal, and a limit is set at $90\%$ confidence level on the branching fraction, $\mathcal{B}(\tau^-\to \mu^-\mu^+\mu^-) < 4.6 \times 10^{-8}$.

Figures and captions

Distribution of (a) $D_s^-$ flight distance and (b) $ {\rm \mathcal{M}_{3body}}$ response for $ D_s^-\rightarrow \phi \left(\mu^+\mu^-\right) \pi^-$ candidates at 8 $\mathrm{\,TeV}$ . The dashed (red) lines indicate the data and the solid (black) lines indicate the simulation. The data is background-subtracted using the sPlot technique [29].

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Distribution of (a) $ {\rm \mathcal{M}_{3body}}$ and (b) $ {\rm \mathcal{M}_{PID}}$ response for 7 $\mathrm{\,TeV}$ data and (c) $ {\rm \mathcal{M}_{3body}}$ and (d) $ {\rm \mathcal{M}_{PID}}$ response for 8 $\mathrm{\,TeV}$ data. The binnings correspond to those used in the extraction of the final results. The short-dashed (red) lines show the response of the data sidebands, whilst the long-dashed (blue) and solid (black) lines show the response of simulated signal events before and after calibration. In all cases the first bin is excluded from the analysis.

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Invariant mass distribution of $\phi(\mu^+\mu^-)\pi^-$ candidates in 8 $\mathrm{\,TeV}$ data. The solid (blue) line shows the overall fit, the long-dashed (green) and short-dashed (red) lines show the two Gaussian components of the $D^-_s$ signal and the dot-dashed (black) line shows the combinatorial background contribution.

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Invariant mass distributions and fits to the mass sidebands in (a) 7 $\mathrm{\,TeV}$ and (b) 8 $\mathrm{\,TeV}$ data for $\mu^+\mu^-\mu^-$ candidates in the bins of $ {\rm \mathcal{M}_{3body}}$ and $ {\rm \mathcal{M}_{PID}}$ response that contain the highest signal probabilities.

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Distribution of $\textrm{CL}_{\textrm{s}}$ values as a function of the assumed branching fraction for $\tau^-\rightarrow \mu^-\mu^+\mu^-$ , under the hypothesis to observe background events only. The dashed line indicates the expected limit and the solid line the observed one. The light (yellow) and dark (green) bands cover the regions of 68% and 95% confidence for the expected limit.

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Animated gif made out of all figures.

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Tables and captions

Terms entering into the normalisation factors, $\alpha$, and their combined statistical and systematic uncertainties.

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Expected background candidate yields in the 7 $\mathrm{\,TeV}$ data set, with their uncertainties, and observed candidate yields within the $\tau^-$ signal window in the different bins of classifier response. The classifier responses range from $0$ (most background-like) to $+1$ (most signal-like). The first bin in each classifier response is excluded from the analysis.

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Expected background candidate yields in the 8 $\mathrm{\,TeV}$ data set, with their uncertainties, and observed candidate yields within the $\tau^-$ signal window in the different bins of classifier response. The classifier responses range from $0$ (most background-like) to $+1$ (most signal-like). The first bin in each classifier response is excluded from the analysis.

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Supplementary Material [file]

Supplementary material full pdf

supple[..].pdf [272 KiB]
supplementary.pdf

This zip file contains supplementary material for the publication LHCb-PAPER-2014-052. The files included are: Supplementary.pdf: The appendix from the paper, including the figures with captions and some brief explanation. *pdf, *eps, *png, *C: The figures

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Created on 09 December 2018.Citation count from INSPIRE on 09 December 2018.