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).
Categories: hep-ex