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

LHCb collaboration, R. Aaij, B. Adeva +797

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 id…