Measurement of the semileptonic decays B¯→Dτ-ν¯τ and B¯→D*τ-ν¯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 sensitive to non-standard model amplitudes in certain scenarios. The data sample consists of 232×106  Υ(4S)→BB̅ decays collected with the BABAR detector at the PEP-II e+e- collider. We select events with a D or D* meson and a light lepton (ℓ=e or μ) recoiling against a fully reconstructed B meson. We perform a fit to the joint distribution of lepton momentum and missing mass squared to distinguish signal B̅ →D(*)τ-ν̅ τ(τ-→ℓ-ν̅ ℓντ) events from the backgrounds, predominantly B̅ →D(*)ℓ-ν̅ ℓ. We measure the branching-fraction ratios R(D)≡B(B̅ →Dτ-ν̅ τ)/B(B̅ →Dℓ-ν̅ ℓ) and R(D*)≡B(B̅ →D*τ-ν̅ τ)/B(B̅ →D*ℓ-ν̅ ℓ) and, from a combined fit to B- and B̅ 0 channels, obtain the results R(D)=(41.6±11.7±5.2)% and R(D*)=(29.7±5.6±1.8)%, where the uncertainties are statistical and systematic. Normalizing to measured B-→D(*)0ℓ-ν̅ ℓ branching fractions, we obtain B(B̅ →Dτ-ν̅ τ)=(0.86±0.24±0.11±0.06)% and B(B̅ →D*τ-ν̅ τ)=(1.62±0.31±0.10±0.05)%, where the additional third uncertainty is from the normalization mode. We also present, for the first time, distributions of the lepton momentum, |pℓ*|, and the squared momentum transfer, q2.