Test of lepton family universality and search for lepton and baryon number violation at Belle

TL;DR

This paper reports measurements of lepton flavor universality and searches for lepton and baryon number violation in B and tau decays using Belle data, providing insights into potential new physics beyond the Standard Model.

Contribution

It presents new measurements of $R_K$ and searches for lepton flavor and baryon number violating decays, expanding the experimental tests of lepton universality and symmetry violations.

Findings

No significant deviation observed in $R_K$ from Standard Model predictions.

No evidence found for lepton flavor violating $B$ decays.

No signals of lepton or baryon number violation in tau decays.

Abstract

The electroweak penguin B decays mediated by $b \to s \ell^+ \ell^- $ transitions are flavour-changing neutral current processes, and are thus sensitive to new physics owing to potential contributions of heavy particles in the quantum loop. Recently, LHCb has obtained interesting results, where the possible hints of lepton family universality violation (LFUV) could be seen. We report a new measurement of the LFUV observable $R_{K}$, the ratio of branching fractions of $B\to K\mu^{+}\mu^{-}$ to $B\to Ke^{+}e^{-}$, based on the full data sample recorded by Belle at the $\Upsilon(4S)$ resonance from $e^+e^-$ collisions produced by the KEKB collider. We also report results on lepton flavor violating $B$ decays, $B^{+}\to K^{+}\mu^{\pm}e^{\mp}$ and $B^{0}\to K_{S}^{0}\mu^{\pm}e^{\mp}$. The $B$-factory at KEK is also a $\tau$ factory, creating a copious amount of $\tau\tau$ pairs. We have used these data to look for lepton and baryon number violating $ \tau$ decays $\tau^{-}\to\overline{p}e^{+}e^{-}$, $pe^{-}e^{-}$, $\overline{p}e^{+}\mu^{-}$, $\overline{p}e^{-}\mu^{+}$, $\overline{p}\mu^{+}\mu^{-}$, and $p\mu^{-}\mu^{-}$. Results of the search are also reported.