Measurement of branching fractions of B decays to K-1(1270)pi and K-1(1400)pi and determination of the CKM angle alpha from B-0 -> a(1)(1260)(+/-)pi(-/+)

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 BB⎯⎯⎯ pairs produced in e+e− annihilation. We measure the branching fractions ℬ(B0→K1(1270)+π−+K1(1400)+π−)=3.1+0.8−0.7×10−5 and ℬ(B+→K1(1270)0π++K1(1400)0π+)=2.9+2.9−1.7×10−5 (<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.