Search for Lepton Flavour Violation in the Decay tau -> mu gamma

TL;DR

This paper reports a search for lepton flavor violation in tau decays to mu gamma, using data from the BABAR experiment, setting an upper limit on the branching ratio due to no observed signal.

Contribution

First search for tau -> mu gamma decay at BABAR, establishing an upper limit on its branching ratio with 221.4/fb of data.

Findings

No significant signal observed.

Upper limit on BR(tau -> mu gamma) < 9 x 10^-8 at 90% CL.

Efficiency of 7.45% for detecting the decay.

Abstract

A search for the lepton flavour violating decay tau -> mu gamma has been performed using 221.4/fb of data collected at an e+e- centre-of-mass energy of 10.58 GeV with the BABAR detector at the PEP-II storage ring. The search has an efficiency of (7.45+/-0.65)% for an expected background level of 6.2+/-0.5 events. In the final sample 4 candidate events are selected. As there is no evidence for a signal in this data, for this preliminary result we set an upper limit of BR(tau -> mu gamma) < 9 x 10^-8 at 90%CL using the method of Feldman and Cousins.