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Measurement of the ratio of branching fractions $\mathcal{B}(B_c^+ \to J/\psi\tau^+\nu_\tau)$/$\mathcal{B}(B_c^+ \to J/\psi\mu^+\nu_\mu)$

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Abstract

A measurement is reported of the ratio of branching fractions $\mathcal{R}(J/\psi)=\mathcal{B}(B_c^+ \to J/\psi\tau^+\nu_\tau)/\mathcal{B}(B_c^+ \to J/\psi \mu^+\nu_\mu)$, where the $\tau^+$ lepton is identified in the decay mode $\tau^+ \to \mu^+\nu_\mu\overline{\nu}_\tau$. This analysis uses a sample of proton-proton collision data corresponding to 3.0$\mathrm{ fb}^{-1}$ of integrated luminosity recorded with the LHCb experiment at center-of-mass energies $7 \mathrm{TeV}$ and $8 \mathrm{TeV}$. A signal is found for the decay $B_c^+ \to J/\psi \tau^+\nu_\tau$ at a significance of 3 standard deviations, corrected for systematic uncertainty, and the ratio of the branching fractions is measured to be $\mathcal{R}(J/\psi) = 0.71 \pm 0.17 \mathrm{ (stat)} \pm 0.18\mathrm{ (syst)}$. This result lies within 2 standard deviations above the range of existing predictions in the Standard Model.

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Distributions of (top) $ m_\mathrm{miss}^2$ , (middle) decay time, and (bottom) $Z$ of the signal data, overlaid with projections of the fit model with all normalization and shape parameters at their best-fit values. Below each panel differences between the data and fit are shown, normalized by the Poisson uncertainty in the data; the dashed lines are at the values $\pm 2$.

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Systematic uncertainties in the determination of $\mathcal{R}( { J \mskip -3mu/\mskip -2mu\psi \mskip 2mu} )$ .

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Supplementary Material [file]

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Created on 11 July 2020.