Measurement of the ratio of the Bc+ -> J/psi tau+ nu_tau and Bc+ -> J/psi mu+ nu_tau branching fractions using three-prong tau lepton decays

The ratio between the 𝐵+𝑐→𝐽/𝜓⁢𝜏+⁢𝜈𝜏 and 𝐵+𝑐→𝐽/𝜓⁢𝜇+⁢𝜈𝜇 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, ℛhad 𝐽/𝜓 =1.0⁢4+0.50 −0.44, is combined with the previous analysis based on the 𝜏+→𝜇+⁢𝜈𝜇⁢¯𝜈 𝜏 leptonic decay channel, leading to ℛ𝐽/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.