Observation of B^0 \to D^{*-} τ^+ ν_τ decay at Belle

Abstract

We report an observation of the decay $B^0\to D^{\ast -}\ddot{I}{„}^{+}\hat{I}{½}_{\ddot{I}}„$ in a data sample containing $535\times 10^6$ $B\bar{B}$ pairs collected with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$ collider. We find a signal with a significance of 5.2 standard deviations and measure the branching fraction $\mathcal{B}(B^0\to D^{\ast -}\ddot{I}{„}^{+}\hat{I}{½}_{\ddot{I}}„)=(2.02_{-0.37}^{+0.40}(stat)\pm 0.37(syst))$. This is the first observation of an exclusive $B$ decay with a $b\to c\ddot{I}„\hat{I}{½}_{\ddot{I}}„$ transition.

6 pages, 3 figures, submitted to Phys. Rev. Lett