Time-integrated and time-dependent angular analyses of B -> J/psi K pi: A measurement of cos2 beta 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 88×10^6 BB̅ 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 cos⁡2β using the angular and time-dependent asymmetry in B→J/ψK*0(KS0π0) decays. With sin⁡2β fixed at its measured value and cos⁡2β treated as an independent parameter, we find cos⁡2β=2.72-0.79+0.50(stat)±0.27(syst), determining the sign of cos⁡2β to be positive at 86% C.L.