Abstract
We present the first evidence of the decay B + → τ + ν τ , using 414 fb −1 of data collected at the ϒ ( 4 S ) 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. The significance of the signal is 3.5 standard deviations including systematics, and the branching fraction is measured to be B ( B + → τ + ν τ ) = ( 1.79 −0.49 +0.56 ( stat ) −0.51 +0.46 ( syst ) ) × 10 −4 . This implies that f B = 0.229 −0.031 +0.036 ( stat ) −0.037 +0.034 ( syst ) GeV ; this is the first direct measurement of this quantity.
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