Observation of the Semileptonic Decays B→D*τ-ν̅ τ and Evidence for B→Dτ-ν̅ τ

We present measurements of the semileptonic decays B-→D0τ-ν̅ τ, B-→D*0τ-ν̅ τ, B̅ 0→D+τ-ν̅ τ, and B̅ 0→D*+τ-ν̅ τ, which are potentially sensitive to non-standard model amplitudes. The data sample comprises 232×106 Υ(4S)→BB̅ decays collected with the BABAR detector. From a combined fit to B- and B̅ 0 channels, we obtain the branching fractions B(B→Dτ-ν̅ τ)=(0.86±0.24±0.11±0.06)% and B(B→D*τ-ν̅ τ)=(1.62±0.31±0.10±0.05)% (normalized for the B̅ 0), where the uncertainties are statistical, systematic, and normalization-mode-related.