Measurement of the semileptonic decays B-bar-->D tau - nu -bar tau and B-bar-->D* tau - nu -bar tau

Abstract

We present measurements of the semileptonic decays B--->D0τ-ν[over-bar] τ, B--->D*0τ-ν[over-bar] τ, B̅ 0-->D+τ-ν̅ τ, and B̅ 0-->D*+τ-ν[over-bar] τ, which are sensitive to non-standard model amplitudes in certain scenarios. The data sample consists of 232×10[superscript 6] Υ(4S)-->BB[over-bar] 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-ν[over-bar] 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 B[over-bar] 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-ν[over-bar] τ)=(0.86±0.24±0.11±0.06)% and B(B[over-bar] -->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, q[superscript 2].