Search for the baryon number and lepton number violating decays $\tau^-\to \Lambda\pi^-$ and $\tau^-\to \bar{\Lambda}\pi^-$ at Belle II

TL;DR

This paper reports a search for rare baryon and lepton number violating tau decays at Belle II, setting new upper limits on their branching fractions due to no observed signal.

Contribution

It provides the first search for these specific baryon and lepton number violating tau decays at Belle II, establishing new upper limits on their branching fractions.

Findings

No evidence of the decay signals was observed.

Upper limits on branching fractions are set at 4.7×10⁻⁸ and 4.3×10⁻⁸.

Data sample of 364 fb⁻¹ was used for the analysis.

Abstract

We present a search for the baryon number $B$ and lepton number $L$ violating decays $\tau^- \rightarrow \Lambda \pi^-$ and $\tau^- \rightarrow \bar{\Lambda} \pi^-$ produced from the $e^+e^-\to \tau^+\tau^-$ process, using a 364 fb$^{-1}$ data sample collected by the Belle~II experiment at the SuperKEKB collider. No evidence of signal is found in either decay mode, which have $|\Delta(B-L)|$ equal to $2$ and $0$, respectively. Upper limits at 90\% credibility level on the branching fractions of $\tau^- \rightarrow \Lambda\pi^-$ and $\tau^- \rightarrow \bar{\Lambda}\pi^-$ are determined to be $4.7 \times 10^{-8}$ and $4.3 \times 10^{-8}$, respectively.