Observation of Large CP Violation and Evidence for Direct CP Violation in B~0 → π~+ π~- Decays

摘要

We report the first observation of CP violation in B~0 → π~+ π~- decays based on 152 X 10~+ Υ(4S) → BB decays collected with the Belle detector at the KEKB asymmetric-energy e~+ e~- collider. We reconstruct a B~0 → 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_(笑?) value.