Search for neutrinoless tau decays tau -> 3l and tau -> l K0S

TL;DR

This paper reports a search for neutrinoless tau decays into three leptons or a lepton and a K0S meson, setting new upper limits on their branching fractions with no evidence of such decays.

Contribution

First search for neutrinoless tau decays into these specific channels using Belle data, establishing more stringent upper limits on their branching fractions.

Findings

No candidate decays observed in any channel.

Set 90% CL upper limits on branching fractions at the 10^{-7} level.

Limits are more stringent than previous measurements.

Abstract

Neutrinoless tau-lepton decays into either three leptons (tau- -> l1- l2 l3) or one lepton and one K0S meson(tau- -> l- K0S) where lepton l means either an electron or muon, have been searched for using 48.6 fb^{-1} of data collected with the Belle detector at the KEKB e+e- collider. No evidence for candidate decays are found in any channel. Therefore we set 90% confidence level upper limits on the branching fraction for 8 different decay modes. These limits are more stringent than those set previously and reach to the 10^{-7} level.