Search for Lepton Flavor Violation in the Decay ${\tau }^{\pm }\to {\mu }^{\pm }\gamma$

A search for the nonconservation of lepton flavor number in the decay ${\tau }^{\pm }\to {\mu }^{\pm }\gamma$ has been performed using $2.07\times {10}^8$ $e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}$ events produced at a center-of-mass energy near 10.58 GeV with the BABAR detector at the PEP-II storage ring. We find no evidence for a signal and set an upper limit on the branching ratio of $\mathcal{B}({\tau }^{\pm }\to {\mu }^{\pm }\gamma )<6.8\times {10}^{-8}$ at 90% confidence level.

Figure 1

NN output shown for data (dots), MC backgrounds (histograms normalized to the luminosity), and MC signal (curves with arbitrary normalization) in the GSB region. Lines with arrows indicate optimized cut positions. The probability of the data-MC ${\chi }^2$ is indicated for each tag mode.

Figure 2

$m_{\mathrm{EC}}$ distribution of data (dots), the background component of the fit (dotted line), and the MC signal (curve with arbitrary normalization) for $|\Delta E-⟨\Delta E⟩|<2\sigma$. The ${\chi }^2$ between data and the background component is 16.0 for 20 bins.