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

TL;DR

This paper reports the first observation of the decay B^0 to D^{*-} tau^+ nu_tau at Belle, with a significance of 5.2 sigma, measuring its branching fraction in a large data sample.

Contribution

It provides the first observation of an exclusive B decay involving a b to c tau nu transition, expanding understanding of semileptonic B decays.

Findings

Signal significance of 5.2 sigma

Measured branching fraction of approximately 2%

First observation of this decay mode

Abstract

We report an observation of the decay $B^0\to D^{*-} \tau^+ \nu_{\tau}$ in a data sample containing $535\times10^6$ $B\bar{B}$ 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)) % $. This is the first observation of an exclusive $B$ decay with a $b \to c \tau \nu_{\tau}$ transition.