Observation of B0 Meson Decay to a1(1260)+- pi-+

We present a measurement of the branching fraction of the decay B0 --> a1(1260)^{+-} pi^{-+} with a1(1260)^{+-} --> pi^{-+} pi^{+-} pi^{+-}. The data sample consists of 218 million B B-bar pairs collected at the Y(4S) resonance and recorded by the BaBar detector at the PEP-II asymmetric-energy e+e- collider at SLAC. The analysis is based on unbinned multivariate maximum likelihood fit. The measured branching fraction is: (16.6 +/- 1.9 +/- 1.5) 10^{-6}. The decay mode, observed for the first time, is seen with a significance of 9.2 sigma (systematic uncertainties included).

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