Observation of Direct CP Violation in B~0 →π~+π~- Decays and Model-Independent Constraints on the Quark-Mixing Angle φ_2

摘要

We report a new measurement of the time-dependent CP-violating parameters in B~0 → π~+π~- decays with 535 x 10~6 BB pairs collected with the Belle detector at the KEKB asymmetric-energy e~+ e~- collider operating at the Y(4S) resonance. We find 1464 ± 65 B~0 → π~+π~- events and measure the CP-violating parameters S_(ππ) = -0.61 ± 0.10(stat) ± 0.04(syst) and A_(ππ) = +0.55 ± 0.08(stat) ± 0.05(syst). We observe large direct CP violation with a significance greater than 5 standard deviations for any S_(ππ) value. Using isospin relations, we measure the Cabibbo-Kobayashi-Maskawa quark-mixing matrix angle φ_2 = (97 ± 11)° for the solution consistent with the standard model and exclude the range 11° < φ_2 < 79° at the 95% confidence level.