Measurement of the Ratio of Branching Fractions B(Bc+→J/ψτ+ντ)/B(Bc+→J/ψμ+νμ)

Abstract

A measurement is reported of the ratio of branching fractions $\mathcal{R}(J/\ensuremath{\psi})=\mathcal{B}({B}_{c}^{+}\ensuremath{\rightarrow}J/\ensuremath{\psi}{\ensuremath{\tau}}^{+}{\ensuremath{\nu}}_{\ensuremath{\tau}})/\phantom{\rule{0ex}{0ex}}\mathcal{B}({B}_{c}^{+}\ensuremath{\rightarrow}J/\ensuremath{\psi}{\ensuremath{\mu}}^{+}{\ensuremath{\nu}}_{\ensuremath{\mu}})$, where the ${\ensuremath{\tau}}^{+}$ lepton is identified in the decay mode ${\ensuremath{\tau}}^{+}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\nu}}_{\ensuremath{\mu}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\tau}}$. This analysis uses a sample of proton-proton collision data corresponding to $3.0\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ of integrated luminosity recorded with the LHCb experiment at center-of-mass energies of 7 and 8 TeV. A signal is found for the decay ${B}_{c}^{+}\ensuremath{\rightarrow}J/\ensuremath{\psi}{\ensuremath{\tau}}^{+}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ at a significance of 3 standard deviations corrected for systematic uncertainty, and the ratio of the branching fractions is measured to be $\mathcal{R}(J/\ensuremath{\psi})=0.71\ifmmode\pm\else\textpm\fi{}0.17(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.18(\mathrm{syst})$. This result lies within 2 standard deviations above the range of central values currently predicted by the standard model.