Search for Lepton Flavor-Violating "tau -> mu gamma" decay

TL;DR

This paper reports a search for the lepton flavor-violating decay tau -> mu gamma using data from the Belle experiment, setting an upper limit on its branching ratio due to no significant signal observed.

Contribution

First search for tau -> mu gamma decay with a large dataset, establishing a new upper limit on its branching ratio.

Findings

Observed one event in the signal region

Expected background was 2.5 ± 0.6 events

Set an upper limit on the branching ratio: Br(tau -> mu gamma) < 6 x 10^{-7}

Abstract

We search for the lepton flavor-violating "tau -> mu gamma" decay using 29.7 million tau pairs accumulated by the Belle experiment. The main background sources are found to be tau pairs with "tau -> mu nu nu" decay and radiative dimuon events. One event is observed in the signal region, while 2.5 +- 0.6 background events are expected. A preliminary upper limit Br(tau -> mu gamma) < 6 x 10^{-7} at the 90% confidence limit is obtained.