Search for the reactions $e^{+}{e}^{-}\to {\mu }^{+}{\tau }^{-}$ and $e^{+}{e}^{-}\to e^{+}{\tau }^{-}$

We report on a search for the lepton-flavor-violating processes $e^{+}{e}^{-}\to {\mu }^{+}{\tau }^{-}$ and $e^{+}{e}^{-}\to e^{+}{\tau }^{-}$. The data sample corresponds to an integrated luminosity of $211{\mathrm{fb}}^{-1}$ recorded by the BABAR experiment at the SLAC PEP-II asymmetric-energy $B$ Factory at a center-of-mass energy of $\sqrt{s}=10.58\mathrm{GeV}$. We find no evidence for a signal and set the 90% confidence level upper limits on the cross sections to be ${\sigma }_{\mu \tau }<3.8\mathrm{fb}$ and ${\sigma }_{e\tau }<9.2\mathrm{fb}$. The ratio of the cross sections with respect to the dimuon cross section are measured to be ${\sigma }_{\mu \tau }/{\sigma }_{\mu \mu }<3.4\times {10}^{-6}$ and ${\sigma }_{e\tau }/{\sigma }_{\mu \mu }<8.2\times {10}^{-6}$.

Figure 1

$m_{\tau }$ versus $p_l^{*}$ for reconstructed candidates for: (a) $e^{+}{e}^{-}\to {\mu }^{+}{\tau }^{-}({\tau }^{-}\to {\pi }^{-}{\pi }^{+}{\pi }^{-}{\nu }_{\tau })$; (b) $e^{+}{e}^{-}\to {\mu }^{+}{\tau }^{-}({\tau }^{-}\to {\pi }^{-}{\nu }_{\tau })$; (c) $e^{+}{e}^{-}\to e^{+}{\tau }^{-}({\tau }^{-}\to {\pi }^{-}{\pi }^{+}{\pi }^{-}{\nu }_{\tau })$; and (d) $e^{+}{e}^{-}\to e^{+}{\tau }^{-}({\tau }^{-}\to {\pi }^{-}{\nu }_{\tau })$.

Figure 2

Reconstructed distributions for $e^{+}{e}^{-}\to {\mu }^{+}{\tau }^{-}$ candidates: (a) $m_{\tau }$ and (b) $p_{\mu }^{*}$ for ${\tau }^{-}\to {\pi }^{-}{\pi }^{+}{\pi }^{-}{\nu }_{\tau }$; and (c) $m_{\tau }$ and (d) $p_{\mu }^{*}$ for ${\tau }^{-}\to {\pi }^{-}{\nu }_{\tau }$. The projection of the ML fit (solid line) hides the background component (dotted line). The projection of the few signal events is shown on the horizontal axis as a dashed line. The peaking dotted line shows the expected MC signal distribution at the 90% CL upper limit.

Figure 3

Reconstructed distributions for $e^{+}{e}^{-}\to e^{+}{\tau }^{-}$ candidates: (a) $m_{\tau }$ and (b) $p_e^{*}$ for ${\tau }^{-}\to {\pi }^{-}{\pi }^{+}{\pi }^{-}{\nu }_{\tau }$; and (c) $m_{\tau }$ and (d) $p_e^{*}$ for ${\tau }^{-}\to {\pi }^{-}{\nu }_{\tau }$. The solid line is the projection of the ML fit, the dotted line is the background component, and the dashed line is the signal component. The peaking dotted line shows the expected MC signal distribution at the 90% CL upper limit.