Evidence of the purely leptonic decay B- → τ-(v)over-barτ

We present the first evidence of the decay B-->tau(-)nu(tau), 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 B(B-->tau(-)nu(tau))=(1.79(-0.49)(+0.56)(stat)(-0.51)(+0.46)(syst))x10(-4). This implies that f(B)=0.229(-0.031)(+0.036)(stat)(-0.037)(+0.034)(syst) GeV and is the first direct measurement of this quantity.