In this Letter, we describe a search for lepton flavor violation (LFV) in the bottomonium system. We search for leptonic decays ${\rm Y}(nS)\to \mu \tau$ ($n=1$, 2, and 3) using the data collected with the CLEO III detector. We identify the $\tau$ lepton using its leptonic decay ${\nu }_{\tau }{\overline{\nu }}_{e}e$ and utilize multidimensional likelihood fitting with probability density function shapes measured from independent data samples. We report our estimates of 95% C.L. upper limits on LFV branching fractions of ${\rm Y}$ mesons. We interpret our results in terms of the exclusion plot for the energy scale of a hypothetical new interaction versus its effective LFV coupling in the framework of effective field theory.

• Received 17 July 2008

DOI:https://doi.org/10.1103/PhysRevLett.101.201601