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

Abstract

We present improved measurements of CP-violation parameters in the decays B[superscript 0]-->π+π-, B[superscript 0]-->K+π-, and B[superscript 0]-->π[superscript 0]π[superscript 0], and of the branching fractions for B[superscript 0]-->π[superscript 0]π[superscript 0] and B[superscript 0]-->K[superscript 0]π[superscript 0]. The results are obtained with the full data set collected at the Υ(4S) resonance by the BABAR experiment at the PEP-II asymmetric-energy B factory at the SLAC National Accelerator Laboratory, corresponding to (467±5)×10[superscript 6] BB̅ pairs. We find the CP-violation parameter values and branching fractions: Sπ+π-=-0.68±0.10±0.03, Cπ+π-=-0.25±0.08±0.02, AK-π+=-0.107±0.016[subscript -0.004][superscript +0.006], Cπ0π0=-0.43±0.26±0.05, B(B0-->π0π0)=(1.83±0.21±0.13)×10-6, B(B[superscript 0]-->K[superscript 0]π[superscript 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 CP violation with a significance of 6.7 standard deviations for B[superscript 0]-->π+π- and 6.1 standard deviations for B0-->K+π-, including systematic uncertainties. Constraints on the unitarity triangle angle α are determined from the isospin relations among the B-->ππ 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.