CP-Violating Asymmetries in B0 --> pi+ pi- Decays With 275 Million BB-bar Pairs

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  • 18 February 2005
Abstract
We present a new measurement of the time-dependent CP-violating parameters in B0 --> pi+ pi- decays using a 253 fb-1 data sample that contains 275 x 10^6 BB-bar 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 666 +/- 43 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: S_pipi = -0.67 +/- 0.16(stat) +/- 0.06(syst) and A_pipi = +0.56 +/- 0.12(stat) +/- 0.06(syst). Large direct CP-violation is observed with a significance greater than 4 standard deviations for any S_pipi value. Using isospin relations, we obtain 95.4% confidence intervals for the CKM angle phi_2 of 0 deg. < phi_2 < 19 deg. and 71 deg. < phi_2 < 180 deg.

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