Search for lepton flavor-violating decay modes $B^0\to K_S^0\tau^\pm\ell^\mp~(\ell=\mu, e)$ with hadronic $B$-tagging at Belle and Belle II

TL;DR

This study searches for rare lepton flavor-violating decays of neutral B mesons into a kaon and a tau-lepton pair, using data from Belle and Belle II, setting new upper limits on their occurrence.

Contribution

First search for $B^0 ightarrow K_S^0 au^ ext{±} ext{l}^ ext{∓}$ decays with hadronic B-tagging at Belle and Belle II, establishing upper limits on branching fractions.

Findings

No evidence of the decay modes was observed.

Upper limits on branching fractions are set between $0.8$ and $3.6 imes 10^{-5}$.

The results constrain new physics models predicting lepton flavor violation.

Abstract

We present the first search for the lepton flavor-violating decay modes $B^0 \rightarrow K_S^0 \tau^\pm \ell^\mp~(\ell=\mu, e)$ using the 711 fb$^{-1}$ and 365 fb$^{-1}$ data samples recorded by the Belle and Belle II detectors, respectively. We use a hadronic $B$-tagging technique, and search for the signal decay in the system recoiling against the fully reconstructed $B$ meson. We find no evidence for $B^0 \rightarrow K_S^0 \tau^\pm \ell^\mp$ decays and set 90\% confidence level upper limits on the branching fractions in the range of $[0.8,\,3.6]\times10^{-5}$.