Measurement of the ratio of the B$_\mathrm{c}^+$ $\to$ J/$ψ$$τ^+ν_τ$ and B$_\mathrm{c}^+$ $\to$ J/$ψ$ $μ^+ν_μ$ branching fractions using three-prong $τ$ lepton decays

Abstract

The ratio between the B$_\mathrm{c}^+$ $\to$ J/$ψ$$τ^+ν_τ$ and B$_\mathrm{c}^+$ $\to$ J/$ψ$$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$/ψ$ $\to$ $μ^+μ^-$ 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, $\mathcal{R}^\text{had}_{\mathrm{J}/ψ}$ = 1.04 $_{-0.44}^{+0.50}$, is combined with the previous analysis based on the $τ^+$ $\to$ $μ^+ν_μ\barν_τ$ leptonic decay channel, leading to $\mathcal{R}_{\mathrm{J}/ψ}$ = 0.49 $\pm$ 0.26. As this result is consistent with the standard model prediction of 0.258 $\pm$ 0.004, no evidence of lepton flavor universality violation is found.