Measurement of the semileptonic decays ¯¯¯B→Dτ−¯ντ and ¯¯¯B→D∗τ−¯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 sensitive to non-standard model amplitudes in certain scenarios. The data sample consists of 232×106Υ(4S)→BB 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 (=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 B→D(*) τ-ντ(τ-→-νντ) events from the backgrounds, predominantly B→D(*)-ν. We measure the branching-fraction ratios R(D)B(B→Dτ-ντ)/ B(B→D-ν) and R(D*)B(B→D*τ- ντ)/B(B→D*-ν) and, from a combined fit to B- and B0 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(*)0- ν branching fractions, we obtain 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)%, where the additional third uncertainty is from the normalization mode. We also present, for the first time, distributions of the lepton momentum, |p*|, and the squared momentum transfer, q2. © 2009 The American Physical Society.