Observation of direct CP violation in B-0 ->pi(+)pi(-) decays and model-independent constraints on the quark-mixing angle phi(2)

Abstract

We report a new measurement of the time-dependent CP-violating parameters in B-0 ->pi(+)pi(-) decays with 535x10(6) B (B) over bar pairs collected with the Belle detector at the KEKB asymmetric-energy e(+)e(-) collider operating at the Upsilon(4S) resonance. We find 1464 +/- 65 B-0 ->pi(+)pi(-) events and measure the CP-violating parameters S-pi pi=-0.61 +/- 0.10(stat)+/- 0.04(syst) and A(pi pi)=+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-pi pi value. Using isospin relations, we measure the Cabibbo-Kobayashi-Maskawa quark-mixing matrix angle phi(2)=(97 +/- 11)degrees for the solution consistent with the standard model and exclude the range 11 degrees <phi(2)< 79 degrees at the 95% confidence level.