Improved Limits on the Lepton-Flavor Violating Decays ${\tau }^{-}\to l^{-}{l}^{+}{l}^{-}$

A search for the neutrinoless, lepton-flavor violating decay of the tau lepton into three charged leptons has been performed using $376{\mathrm{fb}}^{-1}$ of data collected at an $e^{+}{e}^{-}$ center-of-mass energy around 10.58 GeV with the BABAR detector at the SLAC PEP-II storage rings. In all six decay modes considered, the numbers of events found in data are compatible with the background expectations. Upper limits on the branching fractions are set in the range $(4–8)\times {10}^{-8}$ at 90% confidence level.

Figure 1

Observed data shown as dots in the ($\Delta E$, $\Delta M$) plane and the boundaries of the signal region for each decay mode. The dark and light shading indicates contours containing 50% and 90% of the selected MC signal events, respectively.