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

TL;DR

This study searches for lepton flavor-violating decays of B mesons into a K*0, a tau, and an electron or muon, using data from Belle and Belle II, and finds no evidence but sets upper limits on their occurrence.

Contribution

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

Findings

No evidence for the decay modes was observed.

Upper limits on branching fractions are set between 2.9 and 6.4 times 10^{-5}.

Results improve constraints on lepton flavor violation in B decays.

Abstract

We present the results of a search for the charged-lepton-flavor violating decays $B^0 \rightarrow K^{*0}\tau^\pm \ell^{\mp}$, where $\ell^{\mp}$ is either an electron or a muon. The results are based on 365 fb$^{-1}$ and 711 fb$^{-1}$ datasets collected with the Belle II and Belle detectors, respectively. We use an exclusive hadronic $B$-tagging technique, and search for a signal decay in the system recoiling against a fully reconstructed $B$ meson. We find no evidence for $B^0 \rightarrow K^{*0}\tau^\pm \ell^{\mp}$ decays and set upper limits on the branching fractions in the range of $(2.9-6.4)\times10^{-5}$ at 90% confidence level.