| Abstract |
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We present preliminary measurements of branching fractions for the semileptonic decays
B --> D tau nubar and B --> D* tau nubar, which
are potentially sensitive to non--Standard Model amplitudes.
The data sample comprises 232x10^6
Upsilon(4S) --> BBbar decays collected with the BaBar detector at the
PEP-II e+e- storage ring. We obtain
B(B- --> D0 tau nubar) = (0.63 +/- 0.38 +/- 0.10 +/- 0.06)%,
B(B- --> D*0 tau nubar) = (2.35 +/- 0.49 +/- 0.22 +/- 0.18)%,
B(B0bar --> D+ tau nubar) = (1.03 +/- 0.35 +/- 0.14 +/- 0.10)%, and
B(B0bar --> D*+ tau nubar) = (1.15 +/- 0.33 +/- 0.04 +/- 0.04)%,
where the uncertainties
are statistical, systematic, and normalization, respectively.
By combining B- and B0bar results, we also obtain the branching fractions
B(B --> D tau nubar) = (0.90 +/- 0.26 +/- 0.11 +/- 0.06)% and
B(B --> D* tau nubar) = (1.81 +/- 0.33 +/- 0.11 +/- 0.06)%
(quoted for the B- lifetime), with significances
of 3.5 sigma and 6.2 sigma.
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