| Abstract |
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We present searches for the leptonic decays $\Bp\to\ellp\nu$ and the
lepton flavor violating decays $\Bz\to\ell^\pm\tau^\mp$, where $\ell = e,
\mu$, with data collected by the \babar\ experiment at SLAC.
This search demonstrates a novel technique in which we fully reconstruct the accompanying
\Bb in \upsbb events, and look for a monoenergetic lepton from the signal B decay.
The signal yield is extracted from a fit to the signal lepton
candidate momentum distribution in the signal \B rest frame.
Using a data sample of approximately 378 million \BB pairs ($342\,\mbox{fb}^{-1}$), we find
no evidence of signal in any of the decay modes. Branching fraction upper limits of
$\mathcal{B}(\Bp\to\ep\nu)<5.2\times 10^{-6}$, $\mathcal{B}(\Bp\to\mup\nu)<5.6\times 10^{-6}$,
$\mathcal{B}(\Bz\to\ep\taum)<2.8\times 10^{-5}$ and $\mathcal{B}(\Bz\to\mup\taum)<2.2\times 10^{-5}$,
are obtained at 90\% confidence level.
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