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

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

2006-08-11

arxiv.org/abs/hep-ex/0608035v3

Phys.Rev.Lett.98:211801,2007

Physics

High Energy Physics

High Energy Physics - Experiment

10 pages, 5 figures, submitted to ICHEP2006; 6 pages 4 figures, replaced to a paper draft for PRL submission, replaced to a pa

Scientific paper

10.1103/PhysRevLett.98.211801

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

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