| Abstract |
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We present an analysis of exclusive charmless semileptonic B-meson decays
based on 83 million BBbar pairs recorded with the BABAR detector at the
Upsilon(4S) resonance. Using isospin symmetry, we measure branching fractions
B(B0 --> pi- l+ nu) = (1.38 +- 0.10 +- 0.16 +- 0.08) x 10^-4 and
B(B0 --> rho- l+ nu) = (2.14 +- 0.21 +- 0.48 +- 0.28) x 10^-4,
where the errors are statistical, experimental systematic, and
due to form-factor shape uncertainties.
We compare the measured distribution in q^2, the momentum-transfer
squared, with theoretical predictions for the form factors from lattice QCD
and light-cone sum rules, and extract the Cabibbo-Kobayashi-Maskawa (CKM) matrix element
|V_{ub}| = (3.82 +- 0.14 +- 0.22 +- 0.11 +0.88 -0.52) x 10^-3
from B --> pi l nu, where the fourth error reflects the uncertainty of the
form-factor normalization.
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