Cp Violation in B 0 → Π + Π − Decay T. Nakadaira

We present an improved measurement of CP-violating asymmetries in B 0 → π + π − decays based on a 78 fb −1 data sample collected at the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e + e − collider. We reconstruct one neutral B meson as a B 0 → π + π − CP eigenstate and identify the flavor of the accompanying B meson from its decay products. From the asymmetry in the distribution of the time intervals between the two B meson decay points, we obtain the CP-violating asymmetry amplitudes Aππ = +0.77 ±0.27(stat) ±0.08(syst) and Sππ = −1.23 ±0.41(stat) +0.08 −0.07 (syst), where the statistical uncertainties are determined from Monte Carlo pseudo-experiments. We rule out the CP-conserving case, Aππ = Sππ = 0, at the 99.93% confidence level. We discuss how these results constrain the value of the CKM angle φ2.