Search for the lepton flavour violating decays $B^{+} \to K^{+} \tau^\pm \ell^\mp$ ($\ell = e, \mu$) at Belle

TL;DR

This study searches for lepton-flavour-violating decays of B mesons into K and tau lepton pairs, setting the most stringent upper limits to date with no evidence of such decays found.

Contribution

First search for $B^+ o K^+ au^ p ext{ and } au^ p ext{ decays at Belle, establishing the world's best upper limits.

Findings

No evidence for the decays was observed.

Upper limits on branching fractions are in the range of (1-3) x 10^{-5}.

Results set new constraints on lepton-flavour violation in B decays.

Abstract

We present a search for the lepton-flavour-violating decays $B^+ \to K^+ \tau^\pm \ell^\mp$, with $\ell = (e, \mu)$, using the full data sample of $772 \times 10^6$ $B\overline{B}$ pairs recorded by the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. We use events in which one $B$ meson is fully reconstructed in a hadronic decay mode. We find no evidence for $B^\pm \to K^\pm \tau \ell$ decays and set upper limits on their branching fractions at the 90% confidence level in the $(1$-$3) \times 10^{-5}$ range. The obtained limits are the world's best results.