Test of lepton flavor universality in semileptonic B${}_{\text{c}}^{+}$ meson decays in proton-proton collisions at $\sqrt{s}$ = 13 TeV

A measurement of the ratio of branching fractions $R$(J/$\psi$) = $\mathcal{B}$(B${}_{\text{c}}^{+}$ $\to$ J/$\psi$${\tau }^{+}{\nu }_{\tau }$) / $\mathcal{B}$(B${}_{\text{c}}^{+}$ $\to$ J/$\psi$${\mu }^{+}{\nu }_{\mu }$) in the J/$\psi$ $\to$ ${\mu }^{+}{\mu }^{-}$, ${\tau }^{+}$ $\to$ ${\mu }^{+}{\mu }_{\tau }{\bar{\nu }}_{\tau }$ decay channel is presented. This measurement uses a sample of proton-proton collision data collected at a center-of-mass energy of 13 TeV by the CMS experiment in 2018, corresponding to an integrated luminosity of 59.7 fb${}^{-1}$. The measured ratio, $R$(J/$\psi$) = 0.17 ${}_{-0.17}^{+0.18}$ (stat) ${}_{-0.22}^{+0.21}$ (syst) ${}_{-0.18}^{+0.19}$ (theo) = 0.17 $\pm$ 0.33, agrees with the value of 0.2582 $\pm$ 0.0038 predicted by the standard model, which assumes lepton flavor universality. By testing lepton flavor universality, this measurement is a probe of new physics using B${}_{\text{c}}^{+}$ mesons, which are currently only produced at the LHC.