| Abstract |
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We report measurements of the branching fractions of neutral and charged B
meson decays to final states containing a K_1(1270) or K_1(1400) meson and
a charged pion. The data, collected with the BaBar detector at the SLAC
National Accelerator Laboratory, correspond to 454 million B Bbar pairs
produced in e^+ e^- annihilation. We measure the branching fractions
BF(B^0 --> K_1(1270)^+ \pi^- + K_1(1400)^+ \pi^-) = 3.1^{+0.8}_{-0.7} x10^{-5}
and
BF(B^+ --> K_1(1270)^0 \pi^+ + K_1(1400)^0 \pi^+) = 2.9^{+2.9}_{-1.7} x10^{-5}
(<8.2 x10^{-5} at 90\% confidence level), where the errors are statistical
and systematic combined. The B^0 decay mode is observed with a significance
of 7.5\sigma, while a significance of 3.2\sigma is obtained for the B^+ decay
mode. Based on these results, we estimate the weak phase
\alpha = (79 +/- 7 +/- 11)^{\circ} from the time dependent CP asymmetries in
B^0 --> a_1(1260)^{+/-} \pi^{-/+} decays.
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