Observation of B+→a1+(1260)K0 and B0→a1-(1260)K+

We present branching fraction measurements of the decays B+→a1+(1260)K0 and B0→a1-(1260)K+ with a1±(1260)→π∓π±π±. The data sample corresponds to 383×106 BB̅ pairs produced in e+e- annihilation through the Υ(4S) resonance. We measure the products of the branching fractions B(B+→a1+(1260)K0)B(a1+(1260)→π-π+π+)=(17.4±2.5±2.2)×10-6 and B(B0→a1-(1260)K+)B(a1-(1260)→π+π-π-)=(8.2±1.5±1.2)×10-6. We also measure the charge asymmetries Ach(B+→a1+(1260)K0)=0.12±0.11±0.02 and Ach(B0→a1-(1260)K+)=-0.16±0.12±0.01. The first uncertainty quoted is statistical and the second is systematic.