Improved measurements of branching fractions for B -> K pi and B -> pi pi decays

We report improved measurements of branching fractions for B -> K pi and B -> pi pi decays based on a data sample of 449 million B Bbar pairs collected at the Upsilon(4S) resonance with the Belle detector at the KEKB e^+e^- collider. We also calculate the ratios of partial widths for the decays B -> K pi and pi pi, namely R_c = 1.08 +- 0.06 +- 0.08 and R_n = 1.08 +-0.08 +0.09-0.08, where the first and second errors are statistical and systematical, respectively. These ratios are sensitive to enhanced electroweak penguin contributions from new physics; the new measurements are, however, consistent with Standard Model expectations.