A search for the decays $B^0\to l^+\tau^-$ ($l = e,\mu$) using hadronic tag reconstruction

We present a search for the FCNC (and lepton-flavor-violating) decay modes $B^0\to\mu^+\tau^-$ and $B^0\to e^+\tau^-$ with data collected by the BaBar detector at the PEP-II storage ring at SLAC. This search utilizes a technique in which we fully reconstruct the accompanying $\bar{B}^0$ in $\Upsilon(4S)\to B^0\bar{B}^0$ events, and look for a monoenergetic lepton in the $B^0$ frame. The $\tau^-$ is identified by the decay modes $\tau^-\to e^-\nu\bar{\nu}$, $\tau^-\to\mu^-\nu\bar{\nu}$, $\tau^-\to\pi^-\nu$, $\tau^-\to\pi^-\pi^0\nu$, $\tau^-\to\pi^-\pi^0\pi^0\nu$ and $\tau^-\to\pi^-\pi^-\pi^+\nu$. The signal candidates are extracted using a ``cut and count'' method taking advantage of the kinematic variables in the $\tau^-$ rest frame. The data sample consist of approximately 384 million $B\bar{B}$ pairs. The search of rare leptonic decays $B^+\to e^+ \nu_e$ and $B^+\to \mu^+ \nu_\mu$, utilizing the same reconstruction technique, is also discussed and the analysis results are presented.