Weak phase $\alpha$ from $B^0\to a_1^{\pm }(1260){\pi }^{\mp }$

Quasi two-body decays $B^0(t)\to a_1^{\pm }(1260){\pi }^{\mp }$ identified by four charged pions determine a phase ${\alpha }_{\mathrm{eff}}$, which is equal to the weak phase $\alpha$ in the limit of vanishing penguin amplitudes. Applying flavor SU(3) to these decays and to $B\to a_{1}K$ and $B\to K_1\pi$, with $K_1$ an admixture of $K_1(1270)$ and $K_1(1400)$, we derive expressions providing bounds on $\alpha -{\alpha }_{\mathrm{eff}}$. Higher precision in $\alpha$ may be achieved by an overall fit to a complete set of SU(3) related measurements. A method is sketched applying isospin symmetry to time-dependent invariant mass distributions in $B\to {\pi }^{+}{\pi }^{-}{\pi }^0{\pi }^0$.