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

A measurement of the ratio of branching fractions R(J/ψ)=B(Bc+→J/ψτ+ντ)/B(Bc+→J/ψμ+νμ) in the J/ψ→μ+μ−, τ+→μ+νμν¯τ 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/ψ)=0.17−0.17+0.18(stat)−0.22+0.21(syst)−0.18+0.19(theo)=0.17±0.33, agrees with the value of 0.2582±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 Bc+ mesons, which are currently only produced at the LHC.