Search for the Rare Leptonic Decay B- -> tau- nubar

Abstract

We present a search for the decay $B^{-} \to τ^{-} \barν_τ$ in a sample of $88.9 \times 10^{6}$ $B\bar{B}$ pairs recorded with the BaBar detector at the SLAC $B$-Factory. One of the two $B$ mesons from the $Υ(4S)$ is reconstructed in a hadronic or a semileptonic final state and the decay products of the other $B$ in the event are analyzed for consistency with a $B^{-} \to τ^{-} \barν_τ$ decay. We find no evidence of a signal and set an upper limit on the branching fraction of $\mathcal{B}(B^{-} \to τ^{-} \barν_τ) < 4.2 \times 10^{-4}$ at the 90% confidence level.