Measurement of CP asymmetries and branching fractions in charmless two-body B-meson decays to pions and kaons

Abstract

We present improved measurements of C P -violation parameters in the decays B 0 → π + π − , B 0 → K + π − , and B 0 → π 0 π 0 , and of the branching fractions for B 0 → π 0 π 0 and B 0 → K 0 π 0 . The results are obtained with the full data set collected at the Υ ( 4 S ) resonance by the BABAR experiment at the PEP-II asymmetric-energy B factory at the SLAC National Accelerator Laboratory, corresponding to ( 467 ± 5 ) × 10 6 B ¯¯¯ B pairs. We find the C P -violation parameter values and branching fractions: S π + π − = − 0.68 ± 0.10 ± 0.03 , C π + π − = − 0.25 ± 0.08 ± 0.02 , A K − π + = − 0.107 ± 0.016 + 0.006 − 0.004 , C π 0 π 0 = − 0.43 ± 0.26 ± 0.05 , B ( B 0 → π 0 π 0 ) = ( 1.83 ± 0.21 ± 0.13 ) × 10 − 6 , B ( B 0 → K 0 π 0 ) = ( 10.1 ± 0.6 ± 0.4 ) × 10 − 6 , where in each case, the first uncertainties are statistical and the second are systematic. We observe C P violation with a significance of 6.7 standard deviations for B 0 → π + π − and 6.1 standard deviations for B 0 → K + π − , including systematic uncertainties. Constraints on the unitarity triangle angle α are determined from the isospin relations among the B → π π rates and asymmetries. Considering only the solution preferred by the Standard Model, we find α to be in the range [71°,109°] at the 68% confidence level.