Measurement of branching fractions of B decays to K1(1270)π and K1(1400)π and determination of the CKM angle α from B0→a1(1260)±π∓

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We report measurements of the branching fractions of neutral and charged $B$ meson decays to final states containing a ${K}_{1}(1270)$ or ${K}_{1}(1400)$ meson and a charged pion. The data, collected with the BABAR detector at the SLAC National Accelerator Laboratory, correspond to $454\ifmmode\times\else\texttimes\fi{}{10}^{6}$ $B\overline{B}$ pairs produced in ${e}^{+}{e}^{\ensuremath{-}}$ annihilation. We measure the branching fractions $\mathcal{B}({B}^{0}\ensuremath{\rightarrow}{K}_{1}(1270{)}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}+{K}_{1}(1400{)}^{+}{\ensuremath{\pi}}^{\ensuremath{-}})={3.1}_{\ensuremath{-}0.7}^{+0.8}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ and $\mathcal{B}({B}^{+}\ensuremath{\rightarrow}{K}_{1}(1270{)}^{0}{\ensuremath{\pi}}^{+}+{K}_{1}(1400{)}^{0}{\ensuremath{\pi}}^{+})={2.9}_{\ensuremath{-}1.7}^{+2.9}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ ($<8.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ at 90% confidence level), where the errors are statistical and systematic combined. The ${B}^{0}$ decay mode is observed with a significance of $7.5\ensuremath{\sigma}$, while a significance of $3.2\ensuremath{\sigma}$ is obtained for the ${B}^{+}$ decay mode. Based on these results, we estimate the weak phase $\ensuremath{\alpha}=(79\ifmmode\pm\else\textpm\fi{}7\ifmmode\pm\else\textpm\fi{}11)\ifmmode^\circ\else\textdegree\fi{}$ from the time-dependent $CP$ asymmetries in ${B}^{0}\ensuremath{\rightarrow}{a}_{1}(1260{)}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\pi}}^{\ensuremath{\mp}}$ decays.

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