Estimate of SU(3) flavour symmetry breaking in B_s -> K^+ K^- vs. B_d -> pi^+\pi^- decay

I estimate the SU(3) flavour symmetry breaking in the ratio of penguin-to-tree ratios of the decays B_s\to K^+ K^- and B_d\to\pi^+\pi^-, given an assumption on the flavour symmetry breaking of the hadronic input parameters. The decay amplitudes are calculated in QCD factorization. Implications for the determination of gamma are discussed.