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

Abstract

We present a search for the decay $B^{+} \to τ^{+} ν_τ$ in $124.1 \times 10^{6}$ $Υ(4S)$ 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 ($B^- \to D^{*0} \ell^- \barν_{\ell}$) is selected, and in the recoil a search for $B^{+} \to τ^{+} ν_τ$ signal is performed. The $τ$ is identified in the following channels: $τ^+ \to e^+ ν_e \barν_τ$, $τ^+ \to μ^+ ν_μ \barν_τ$, $τ^+ \to π^+ \barν_τ$, $τ^+ \to π^+ π^{0} \barν_τ$, $τ^+ \to π^+ π^{-} π^{+} \barν_τ$. We find no evidence of signal, and we set a preliminary upper limit on the branching fraction of $\mathcal{B}(B^{+} \to τ^{+} ν_τ) < 4.3 \times 10^{-4}$ at the 90% confidence level (CL). This result is then combined with a statistically independent $BABAR$ search for $B^{+} \to τ^{+} ν_τ$ to give a combined preliminary limit of $\mathcal{B}(B^{+} \to τ^{+} ν_τ) < 3.3 \times 10^{-4}$ at 90% CL.