Abstract

We present the first evidence for B+ → τ+ντ decay, using 414 fb−1 of B meson decay event sample collected with the Belle detector at the KEKB e+e− collider. To cope with large missing energy due to multiple neutrinos in the final state, events are tagged by fully reconstructing one of the B mesons. We find the evidence for signal with a significance of 3.5 σ including systematic uncertainties. The branching fraction is measured to be B(B+ → τ+ντ) = (1.79−0.49−0.51+0.56+0.46) × 10−4. From this we obtain fB = 0.229−0.031−0.037+0.036+0.034 GeV, the first direct determination of the B meson decay constant.

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