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 $B^0\to {\pi }^{+}{\pi }^{-}$, $B^0\to K^{+}{\pi }^{-}$, and $B^0\to {\pi }^0{\pi }^0$, and of the branching fractions for $B^0\to {\pi }^0{\pi }^0$ and $B^0\to K^0{\pi }^0$. The results are obtained with the full data set collected at the ${\rm Y}(4S)$ resonance by the BABAR experiment at the PEP-II asymmetric-energy $B$ factory at the SLAC National Accelerator Laboratory, corresponding to $467\pm 5$ million $B\bar{B}$ pairs. We find the CP-violation parameter values and branching fractions $S_{{\pi }^{+}{\pi }^{-}}=-0.68\pm 0.10\pm 0.03,C_{{\pi }^{+}{\pi }^{-}}=-0.25\pm 0.08\pm 0.02,A_{K^{-}{\pi }^{+}}=-0.107\pm 0.016_{-0.004}^{+0.006},C_{{\pi }^0{\pi }^0}=-0.43\pm 0.26\pm 0.05,Br(B^0\to {\pi }^0{\pi }^0)=(1.83\pm 0.21\pm 0.13)\times 10^{-6},Br(B^0\to K^0{\pi }^0)=(10.1\pm 0.6\pm 0.4)\times 10^{-6},$ 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 $B^0\to {\pi }^{+}{\pi }^{-}$ and 6.1 standard deviations for $B^0\to K^{+}{\pi }^{-}$, including systematic uncertainties. Constraints on the Unitarity Triangle angle $\alpha$ are determined from the isospin relations among the $B\to \pi \pi$ rates and asymmetries. Considering only the solution preferred by the Standard Model, we find $\alpha$ to be in the range $[71^{\circ },109^{\circ }]$ at the 68% confidence level.

18 pages, 11 postscript figures, submitted to Phys. Rev. D

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