Measurement of the ratio of the B c + → J / ψ τ + ν τ and B c + → J / ψ μ + ν μ branching fractions using three-prong τ lepton decays

The ratio between the B c + → J / ψ τ + ν τ and B c + → J / ψ μ + ν μ branching fractions is measured using a data sample of proton-proton collisions collected by CMS at a center-of-mass energy of 13 TeV in the years 2016–2018 and corresponding to an integrated luminosity of 138 fb − 1 . The J / ψ meson is identified through its J / ψ → μ + μ − decay and the tau lepton is reconstructed in the hadronic three-prong final state. The measured ratio of branching fractions in this tau decay mode, R J / ψ had = 1.0 4 − 0.44 + 0.50 , is combined with the previous analysis based on the τ + → μ + ν μ ν ¯ τ leptonic decay channel, leading to R J / ψ = 0.49 ± 0.26 . As this result is consistent with the standard model prediction of 0.258 ± 0.004 , no evidence of lepton flavor universality violation is found.