A measurement is reported of the ratio of branching fractions $mathcal{R}(J/psi)=mathcal{B}(B_c^+,to,J/psitau^+ u_tau)/mathcal{B}(B_c^+,to,J/psi mu^+ u_mu)$, where the $tau^+$ lepton is identified in the decay mode $tau^+,to,mu^+ u_muoverline{ u}_tau$. 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 $B_c^+,to,J/psi tau^+ u_tau$ at a significance of 3 standard deviations, corrected for systematic uncertainty, and the ratio of the branching fractions is measured to be $mathcal{R}(J/psi) = 0.71 pm 0.17 mathrm{,(stat)} pm 0.18mathrm{,(syst)}$. This result lies within 2 standard deviations above the range of existing predictions in the Standard Model.
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