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

We report an observation of the decay $B^0\to D^{\ast -}{\tau }^{+}{\nu }_{\tau }$ 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^{*-} \tau ^+ \nu_{\tau})=(2.02 ^{+0.40}{-0.37} (stat) \pm 0.37 (syst)) % $\mathrm{.}Thisisthefirstobservationofanexclusive$B$decaywitha$b \to c \tau \nu{\tau}$ transition.