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We present the studies of the decays $B\to a_1(1260) \pi$ and $a_1(1260) K$within the framework of QCD factorization. Due to the G-parity, unlike thevector meson, the chiral-odd two-parton light-cone distribution amplitudes ofthe $a_1$ are antisymmetric under the exchange of quark and anti-quark momentumfractions in the SU(2) limit. The branching ratios for $a_1 \pi$ modes aresensitive to tree--penguin interference. The resultant ${\cal B}(B^0 \toa_1^\pm \pi^\mp)$ are in good agreement with the data. However, using thecurrent Cabibbo--Kobayashi--Maskawa angles, $\beta=22.0^\circ$ and$\gamma=59.0^\circ$, our results for the mixing-induced parameter $S$ and$\alpha_{\rm eff}$ differ from the measurements of the time-dependent CPasymmetries in the decay $B^0\to a_1^\pm \pi^\mp$ at about the $3.7\sigma$level. This puzzle may be resolved by using a larger $\gamma \gtrsim 80^\circ$.For $a_1 K$ modes, the annihilation topologies give sizable contributions andare sensitive to the first Gegenbauer moment of the leading-twist tensor(chiral-odd) distribution amplitude of the $a_1$ meson. The $B\to a_1 K$amplitudes resemble the corresponding $B\to \pi K$ ones very much. Taking theratios of corresponding CP-averaged $a_1 K$ and $\pi K$ branching ratios, wecan extract information relevant to the electroweak penguins and annihilations.The existence of new-physics in the electroweak penguin sector and final stateinteractions during decays can thus be explored.

Branching Ratios and CP Asymmetries of B \to a_1(1260) \pi and a_1(1260) K Decays