Observation of Direct CP-Violation in B0 --> pi+pi- Decays and Model-Independent Constraints on phi2

Abstract

We present a new measurement of the time-dependent CP-violating parameters in B0 -> pi+pi- decays using a data sample that contains 535M BBbar pairs collected with the Belle detector at the KEKB asymmetric-energy e+e- collider operating at the Upsilon(4S) resonance. We reconstruct one neutral B meson as a pi+pi- CP eigenstate and identify the flavor of the accompanying B meson from its decay products. We find 1464+-65 B0 --> pi+pi- events and perform an unbinned maximum likelihood fit to the distribution of the proper-time differences between the two B meson decays. The fit yields the CP-violating parameters: Spipi = -0.61 +- 0.10(stat) +- 0.04(syst) and Apipi = +0.55 +- 0.08(stat) +- 0.05(syst). Large direct CP-violation is observed with a significance greater than 5 standard deviations for any Spipi value. Using isospin relations, we measure the Cabibbo-Kobayashi-Maskawa quark-mixing matrix angle phi2 = (97 +- 11) degrees for the solution consistent with the standard model and exclude a range of 11<phi2<79 degrees at the 95% confidence level.