Improved Evidence for Direct $CP$ Violation in ${B}^{0}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ Decays and Model-Independent Constraints on ${\ensuremath{\phi}}_{2}$

Improved Evidence for Direct $C P$ Violation in $B^{0} \to π^{+} π^{-}$ Decays and Model-Independent Constraints on $ϕ_{2}$

K. Abe et al. (Belle Collaboration)

Phys. Rev. Lett. 95, 101801 – Published 31 August 2005

Abstract

We present a new measurement of the time-dependent $C P$ -violating parameters in $B^{0} \to π^{+} π^{-}$ decays with $275 \times 10^{6}$ $B \bar{B}$ pairs collected with the Belle detector at the KEKB asymmetric-energy $e^{+} e^{-}$ collider operating at the $Υ (4 S)$ resonance. We find $666 \pm 43$ $B^{0} \to π^{+} π^{-}$ events and measure the $C P$ -violating parameters: $S_{π π} = - 0.67 \pm 0.16 (stat) \pm 0.06 (syst)$ and $A_{π π} = + 0.56 \pm 0.12 (stat) \pm 0.06 (syst)$ . We find evidence for large direct $C P$ violation with a significance greater than 4 standard deviations for any $S_{π π}$ value. Using isospin relations, we obtain 95.4% confidence intervals for the Cabibbo-Kobayashi-Maskawa quark-mixing matrix angle $ϕ_{2}$ of $0 ° < ϕ_{2} < 19 °$ and $71 ° < ϕ_{2} < 180 °$ .