Measurement of branching fractions of B decays to K_{1}(1270)π and K_{1}(1400)π and determination of the CKM angle α from B^{0}→a_{1}(1260)^{±}π^{∓}

We report measurements of the branching fractions of neutral and charged B meson decays to final states containing a K1(1270) or K1(1400) meson and a charged pion. The data, collected with the BABAR detector at the SLAC National Accelerator Laboratory, correspond to 454×106 BB̅ pairs produced in e+e- annihilation. We measure the branching fractions B(B0→K1(1270)+π-+K1(1400)+π-)=3.1-0.7+0.8×10-5 and B(B+→K1(1270)0π++K1(1400)0π+)=2.9-1.7+2.9×10-5 (<8.2×10-5 at 90% confidence level), where the errors are statistical and systematic combined. The B0 decay mode is observed with a significance of 7.5σ, while a significance of 3.2σ is obtained for the B+ decay mode. Based on these results, we estimate the weak phase α=(79±7±11)° from the time-dependent CP asymmetries in B0→a1(1260)±π∓ decays.