Searches for Lepton Flavor Violation in the Decays ${\tau }^{\pm }\to e^{\pm }\gamma$ and ${\tau }^{\pm }\to {\mu }^{\pm }\gamma$

Searches for lepton-flavor-violating decays of a $\tau$ lepton to a lighter mass lepton and a photon have been performed with the entire data set of $(963\pm 7)\times {10}^6\tau$ decays collected by the BABAR detector near the ${\rm Y}(4S)$, ${\rm Y}(3S)$ and ${\rm Y}(2S)$ resonances. The searches yield no evidence of signals and we set upper limits on the branching fractions of $\mathcal{B}({\tau }^{\pm }\to e^{\pm }\gamma )<3.3\times {10}^{-8}$ and $\mathcal{B}({\tau }^{\pm }\to {\mu }^{\pm }\gamma )<4.4\times {10}^{-8}$ at 90% confidence level.

Figure 1

The GSB and the $2\sigma$ ellipse for ${\tau }^{\pm }\to e^{\pm }\gamma$ (left) and ${\tau }^{\pm }\to {\mu }^{\pm }\gamma$ (right) decays in the $m_{\mathrm{EC}}$ vs $\Delta E$ plane. Data are shown as dots and contours containing 90% (50%) of signal MC events are shown as light- (dark-) shaded regions.