Time-integrated and time-dependent angular analyses of B→J/ψKπ: A measurement of cos2β with no sign ambiguity from strong phases

We present results on B→J/ψKπ decays using e+e-annihilation data collected with the BABAR detector at the Υ(4S) resonance. The detector is located at the PEP-II asymmetric-energy storage ring facility at the Stanford Linear Accelerator Center. Using approximately 88x106 BB pairs, we measure the decay amplitudes for the flavor eigenmodes and observe strong-phase differences indicative of final-state interactions with a significance of 7.6 standard deviations. We use the interference between the Kπ S-wave and P-wave amplitudes in the region of the K(892) to resolve the ambiguity in the determination of these strong phases. We then perform an ambiguity-free measurement of cos2β using the angular and time-dependent asymmetry in B→J/ψK0(KS0π0) decays. With sin2β fixed at its measured value and cos2β treated as an independent parameter, we find cos2β=2.72-0.79+0.50(stat)±0.27(syst), determining the sign of cos2β to be positive at 86% C.L