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

A measurement of the ratio of branching fractions $ R(J/\psi )=\mathcal{B}({B}_{c}^{+}\to J/\psi {\tau }^{+}{\nu }_{\tau })/\mathcal{B}({B}_{c}^{+}\to J/\psi {\mu }^{+}{\nu }_{\mu })$ in the $ J/\psi \to {\mu }^{+}{\mu }^{-}$, $ {\tau }^{+}\to {\mu }^{+}{\nu }_{\mu }{\overline{\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\text{}\text{}\mathrm{TeV}$ by the CMS experiment in 2018, corresponding to an integrated luminosity of $ 59.7\text{}\text{}{\mathrm{fb}}^{-1}$. The measured ratio, $ R(J/\psi )=\phantom{\rule{0ex}{0ex}}0.1{7}_{-0.17}^{+0.18}{(\mathrm{stat})}_{-0.22}^{+0.21}{(\mathrm{syst})}_{-0.18}^{+0.19}(\mathrm{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}_{c}^{+}$ mesons, which are currently only produced at the LHC.