Measurement of the ratio of the B$_\mathrm{c}^+$ $\to$ J/$\psi$$\tau^+\nu_\tau$ and B$_\mathrm{c}^+$ $\to$ J/$\psi$ $\mu^+\nu_\mu$ branching fractions using three-prong $\tau$ lepton decays

TL;DR

This paper measures the ratio of specific Bc+ meson decay branching fractions involving tau and muon leptons, using CMS data, and finds results consistent with the Standard Model, showing no lepton flavor universality violation.

Contribution

First measurement of the Bc+ decay ratio involving tau and muon leptons using three-prong tau decays with CMS data, combining hadronic and leptonic channels for improved accuracy.

Findings

Measured ratio in tau decay mode: 1.04_{-0.44}^{+0.50}

Combined ratio with previous analysis: 0.49 ± 0.26

Results are consistent with Standard Model prediction

Abstract

The ratio between the B$_\mathrm{c}^+$ $\to$ J/$\psi$$\tau^+\nu_\tau$ and B$_\mathrm{c}^+$ $\to$ J/$\psi$$\mu^+\nu_\mu$ 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}$. The J$/\psi$ meson is identified through its J$/\psi$ $\to$ $\mu^+\mu^-$ 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, $\mathcal{R}^\text{had}_{\mathrm{J}/\psi}$ = 1.04$_{-0.44}^{+0.50}$, is combined with the previous analysis based on the $\tau^+$ $\to$ $\mu^+\nu_\mu\bar{\nu}_\tau$ leptonic decay channel, leading to $\mathcal{R}_{\mathrm{J}/\psi}$ = 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.