| Abstract |
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We report preliminary measurements of the charmless exclusive semileptonic
branching fractions of the B0 --> pi- l+ nu and B+ --> pi0 l+ nu decays,
based on 211 fb-1 of data collected at the Y(4S) resonance by the BABAR
detector. In events in which the decay of one B meson to a hadronic final
state is fully reconstructed, the semileptonic decay of the second B meson
is identified by the detection of a charged lepton and a pion.
We measure the partial branching fractions for B0 --> pi- l+ nu and
B+ --> pi0 l+ nu in three regions of the invariant mass squared of the lepton
pair, and we obtain the total branching fractions
BF(B0 --> pi- l+ nu) = (1.14 +/- 0.27(stat) +/- 0.17(syst)) x 10^-4 and
BF(B+ --> pi0 l+ nu) = (0.86 +/- 0.22(stat) +/- 0.11(syst)) x 10^-4.
Using isospin symmetry, we measure the combined total branching fraction
BF(B0 --> pi- l+ nu) = (1.28 +/- 0.23(stat) +/- 0.16(syst)) x 10^-4.
Theoretical predictions of the form-factor are used to determine the magnitude
of the Cabibbo-Kobayashi-Maskawa matrix element
|V_ub| = (3.7 +/- 0.3(stat) +/- 0.2(syst) +0.8-0.5(FF)) x 10^-3,
where the last error is due to the form-factor normalization.
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