Search for Lepton Flavour Violating Decays $\tau \to \ell K^0_S$ with the BaBar Experiment

TL;DR

This paper reports a search for lepton flavor violating decays of tau leptons into a lighter lepton and a neutral kaon, using BaBar data, setting upper limits on the decay probabilities due to no significant signal detection.

Contribution

First search for $ au o ext{lepton} + K^0_S$ decays using BaBar data, establishing new upper limits on their branching fractions.

Findings

No significant signal observed in either decay channel.

Upper limits on branching fractions: $3.3 imes 10^{-8}$ for $ au o e K^0_S$ and $4.0 imes 10^{-8}$ for $ au o \mu K^0_S$.

Results constrain models predicting lepton flavor violation.

Abstract

A search for the lepton flavour violating decays $\tau^- \to l^- K^0_{\scriptscriptstyle S}$ ($l = e$ or $\mu$) has been performed using a data sample corresponding to an integrated luminosity of 469 ${fb}^{-1}$, collected with the {\slshape B\kern-0.1em{\smaller A}\kern-0.1em B\kern-0.1em{\smaller A\kern-0.2em R}} detector at the SLAC PEP-II $e^+e^-$ asymmetric energy collider. No statistically significant signal has been observed in either channel and the estimated upper limits on branching fractions are ${\cal B}(\tau^- \to e^- K^0_{\scriptscriptstyle S}) < 3.3 \times 10^{-8}$ and ${\cal B}(\tau^- \to \mu^- K^0_{\scriptscriptstyle S}) < 4.0 \times 10^{-8}$ at 90% confidence level.