Improved Evidence for Direct CP Violation in B0 --> pi+ pi- Decays and Model-Independent Constraints on phi_2

TL;DR

This paper reports a new measurement of CP violation in B0 to pi+ pi- decays, providing evidence for direct CP violation and constraining the CKM angle phi_2 using a large data sample from the Belle experiment.

Contribution

It presents the first evidence for large direct CP violation in B0 to pi+ pi- decays and offers model-independent constraints on the CKM angle phi_2.

Findings

Evidence for large direct CP violation with >4 sigma significance.

Measured CP-violating parameters: S_pipi and A_pipi.

95.4% confidence intervals for phi_2: 0-19° and 71-180°.

Abstract

We present a new measurement of the time-dependent CP-violating parameters in B0 --> pi+ pi- decays with 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 find 666 +/- 43 B0 --> pi+ pi- events and measure 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). We find evidence for large direct CP-violation 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.