Limitations in measuring the angle β by using SU(3) relations for B-meson decay-amplitudes

Flavour SU(3) symmetry of strong interactions and certain dynamical assumptions have been used in a series of recent publications to extract weak CKM phases from B-decays into {ππ, πK, KK̄} final states. We point out that irrespectively of SU(3)-breaking effects the presence of QCD-penguin contributions with internal u- and c-quarks precludes a clean determination of the angle β in the unitarity triangle by using the branching ratios only. This difficulty can be overcome by measuring in addition the ratio xd/xs of Bd0−B̄d0 to Bs0−B̄s0 mixings. The measurement of the angle γ is unaffected by these new contributions. Some specific uncertainties related to SU(3)-breaking effects and electroweak penguin contributions are briefly discussed.