We present measurements of the branching fraction $\mathcal{B}$ and longitudinal polarization fraction $f_L$ for $B^0\to a_1(1260{)}^{+}{a}_1(1260{)}^{-}$ decays, with $a_1(1260{)}^{\pm }\to {\pi }^{-}{\pi }^{+}{\pi }^{\pm }$. The data sample, collected with the BABAR detector at the SLAC National Accelerator Laboratory, represents $465\times {10}^6$ produced $B\overline{B}$ pairs. We measure $\mathcal{B}(B^0\to a_1(1260{)}^{+}{a}_1(1260{)}^{-})\times [\mathcal{B}(a_1(1260{)}^{+}\to {\pi }^{-}{\pi }^{+}{\pi }^{+}){]}^2=(11.8\pm 2.6\pm 1.6)\times {10}^{-6}$ and $f_L=0.31\pm 0.22\pm 0.10$, where the first uncertainty is statistical and the second systematic. The decay mode is measured with a significance of 5.0 standard deviations including systematic uncertainties.

• Received 13 July 2009

DOI:https://doi.org/10.1103/PhysRevD.80.092007