Evidence of the Purely Leptonic Decay B- --> tau- nu_tau-bar

TL;DR

This paper reports the first evidence of the purely leptonic decay B- --> tau- nu_tau-bar, measuring its branching fraction and extracting the decay constant f_B using data from the Belle experiment.

Contribution

First experimental evidence and measurement of the B- --> tau- nu_tau-bar decay and the decay constant f_B using a large data sample from Belle.

Findings

Branching fraction measured as (1.79 +0.56-0.49(stat) +0.46-0.51(syst))*10^-4

Significance of the signal is 3.5 standard deviations including systematics

First direct measurement of the decay constant f_B as 0.229 +0.036-0.031(stat) +0.034-0.037(syst) GeV

Abstract

We present the first evidence of the decay B- --> tau- nu_tau-bar using 414 fb^-1 of data collected at the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. Events are tagged by fully reconstructing one of the B mesons in hadronic modes. We detect the signal with a significance of 3.5 standard deviations including systematics, and measure the branching fraction to be Br(B- --> tau- nu_tau-bar) = (1.79 +0.56-0.49(stat) +0.46-0.51(syst))*10^-4. This implies that f_B = 0.229 +0.036-0.031(stat) +0.034-0.037(syst) GeV and is the first direct measurement of this quantity.