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

A search for the nonconservation of lepton flavor in the decay ${\tau }^{\pm }\to e^{\pm }\gamma$ has been performed with $2.07\times {10}^8$ $e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}$ events collected by the BABAR detector at the SLAC PEP II storage ring at a center-of-mass energy near 10.58 GeV. We find no evidence for a signal and set an upper limit on the branching ratio of $\mathcal{B}({\tau }^{\pm }\to e^{\pm }\gamma )<1.1\times {10}^{-7}$ at 90% confidence level.

Figure 1

$m_{\mathrm{EC}}$ vs $\Delta E$ distribution of data (dots) and shaded region containing 50% of the selected signal MC events inside the grand signal box, as defined in the text. The boundary of the $\pm 2\sigma$ signal box is also shown.

Figure 2

$m_{\mathrm{EC}}$ distribution of data (dots), the expected backgrounds (histograms), and MC signal (curve with arbitrary normalization) for $|\Delta E-⟨\Delta E⟩|<2\sigma (\Delta E)$.

Figure 3

Upper limits of 90% C.L. on $M_{\tilde{L}13}^2/M_{\tilde{L}11}^2$ for $\mathcal{B}({\tau }^{\pm }\to e^{\pm }\gamma )<1.1\times {10}^{-7}$ with $\tan \beta =10$, 20, and 40.