Evidence for CP-Violating Asymmetries in B0->pi+pi- Decays and Constraints on the CKM Angle phi2

TL;DR

This paper reports an improved measurement of CP-violating asymmetries in B0 -> pi+ pi- decays, providing evidence against CP conservation and constraining the CKM angle phi2 using data from the Belle detector.

Contribution

It presents the most precise measurement to date of CP asymmetries in B0 -> pi+ pi- decays and discusses implications for the CKM angle phi2.

Findings

CP asymmetry amplitudes Apipi = +0.77±0.27(stat)±0.08(syst)

Spipi = -1.23±0.41(stat)±0.07(syst)

CP-conserving case ruled out at 99.93% confidence level

Abstract

We present an improved measurement of CP-violating asymmetries in B0 -> pi+ pi- decays based on a 78 fb^-1 data sample collected at the Y(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 inclusive properties of 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 = +0.77+/-0.27(stat)+/-0.08(syst) and Spipi = -1.23+/-0.41(stat)+0.08/-0.07(syst), where the statistical uncertainties are determined from Monte Carlo pseudo-experiments. We obtain confidence intervals for CP-violating asymmetry parameters Apipi and Spipi based on a frequentist approach. We rule out the CP-conserving case, Apipi=Spipi=0, at the 99.93% confidence level. We discuss how these results constrain the value of the CKM angle phi2.