Observation of Large CP Violation and Evidence for Direct CP Violation in B0→π+π- Decays

We report the first observation of CP violation in B⁰→π⁺π^- decays based on 152×10⁶ υ(4S)→BB¯ decays collected with the Belle detector at the KEKB asymmetric-energy e⁺e^- collider. We reconstruct a B⁰→π⁺π^- CP eigenstate and identify the flavor of the accompanying B meson from its decay products. From the distribution of the time intervals between the two B meson decay points, we obtain A_ππ=+0.58±0.15(stat)±0.07(syst) and S_ππ=-1.00±0.21(stat)±0.07(syst). We rule out the CP-conserving case, A_ππ=S_ππ=0, at a level of 5.2 standard deviations. We also find evidence for direct CP violation with a significance at or greater than 3.2 standard deviations for any S_ππ value.