Observation of Large CP Violation and Evidence for Direct CP Violation in B0-->pi+pi- Decays

TL;DR

This paper reports the first observation of CP violation in B0 to pi+pi- decays, providing evidence for direct CP violation with high statistical significance, based on data collected at the Belle detector.

Contribution

It presents the first measurement of CP-violating asymmetries in B0 to pi+pi- decays, demonstrating direct CP violation with significant statistical confidence.

Findings

Measured CP asymmetry amplitudes Apipi and Spipi with uncertainties.

Ruled out CP conservation at 5.2 sigma level.

Provided evidence for direct CP violation at ≥3.2 sigma.

Abstract

We report the first observation of CP-violating asymmetries in B0 --> pi+pi- decays based on a 140 fb-1 data sample collected at the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. We reconstruct one neutral B meson as a B0 --> pi+pi- CP eigenstate and identify the flavor of the accompanying B meson from its decay products. We apply an unbinned maximum likelihood fit to the distribution of the time intervals between the two B meson decay points. The fit yields the CP-violating asymmetry amplitudes Apipi = +0.58+/-0.15(stat)+/-0.07(syst) and Spipi = -1.00+/-0.21(stat)+/-0.07(syst). We rule out the CP-conserving case, Apipi=Spipi=0, at a level of 5.2 standard deviations. We also find evidence for direct CP violation with a significance at or greater than 3.2 standard deviations for any Spipi value.