Observation of B^0 -> a_1^{+/-}(1260) \pi^{-/+}

We report a new measurement of the branching fraction of neutral $B$ meson decays $B^0\ra a_{1}^{\pm}(1260)\pi^{\mp}$ with \ensuremath{a_{1}^{\pm}(1260)\ra \rho^{0}\pi^{\pm}} and $\rho^0\ra pi^+\pi^-$ using $275\times 10^{6}$ BBar pairs collected by the Belle detector at the KEKBasymmetric-energy $e^{+}e^{-}$ collider. We measure the branching fraction $\mathcal{B}($B^0\ra a_{1}^{\pm}(1260)\pi^{\mp}=(48.6\pm 4.1(stat)\pm 3.9(syst)) \times 10^{-6}$. Using a relativistic Breit-Wigner parameterization, we measure the mass and width of the$a_{1}(1260)$to be$m_{a_1}=1197\pm 34 MeV/c^2$and$\Gamma_{a_1}=305\pm 43 MeV/c^2$, respectively.