Observation of the Semileptonic Decays B→D∗τ−¯ντ and Evidence for B→Dτ−¯ντ

We present measurements of the semileptonic decays B−→D0τ−¯ντ, B−→D*0τ−¯ντ, ¯¯¯B0→D+τ−¯ντ, and ¯¯¯B0→D*+τ−¯ντ, which are potentially sensitive to non-standard model amplitudes. The data sample comprises 232×10 6 Υ(4S)→B¯¯¯B decays collected with the BABAR detector. From a combined fit to B− and ¯¯¯B0 channels, we obtain the branching fractions B(B→Dτ−¯ντ)=(0.86±0.24±0.11±0.06)% and B(B→D∗τ−¯ντ)=(1.62±0.31±0.10±0.05)% (normalized for the ¯¯¯B0), where the uncertainties are statistical, systematic, and normalization-mode-related.