A Search for B --> tau nu Recoiling against B --> D*0 l nu

Abstract

We present a search for the decay $B^{+} \to \tau^{+} \nu_{\tau}$ in $124.1 \times 10^{6}$ $\Upsilon(4S)$ decays recorded with the $BABAR$ detector at the SLAC PEP-II $B$-Factory. A sample of events with one reconstructed exclusive semi-leptonic B decay ($B^- \to D^{*0} \ell^- \bar{\nu}_{\ell}$) is selected, and in the recoil a search for $B^{+} \to \tau^{+} \nu_{\tau}$ signal is performed. The $\tau$ is identified in the following channels: $\tau^+ \to e^+ \nu_e \bar{\nu}_{\tau}$, $\tau^+ \to \mu^+ \nu_{\mu} \bar{\nu}_{\tau}$, $\tau^+ \to \pi^+ \bar{\nu}_{\tau}$, $\tau^+ \to \pi^+ \pi^{0} \bar{\nu}_{\tau}$, $\tau^+ \to \pi^+ \pi^{-} \pi^{+} \bar{\nu}_{\tau}$. We find no evidence of signal, and we set a preliminary upper limit on the branching fraction of $\mathcal{B}(B^{+} \to \tau^{+} \nu_{\tau}) < 4.3 \times 10^{-4}$ at the 90% confidence level (CL). This result is then combined with a statistically independent $BABAR$ search for $B^{+} \to \tau^{+} \nu_{\tau}$ to give a combined preliminary limit of $\mathcal{B}(B^{+} \to \tau^{+} \nu_{\tau}) < 3.3 \times 10^{-4}$ at 90% CL.