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We investigate to what extent the recently measured value for a non-vanishingdirect CP asymmetry in D0 -> K+ K- and D0 -> pi+ pi- decays can be accommodatedin the Standard Model (SM) or extensions with a constrained flavour sector, forinstance from a sequential 4th generation of quarks (4G). From the comparisonwith D0 -> K- pi+ branching ratios, we establish large U-spin symmetry (d <->s) breaking effects with large strong phases between different interferingamplitudes. On the basis of conservative estimates on amplitude ratios - whichare supported by an analysis of the breaking of a (c <-> u) symmetry innon-leptonic B0 decays - we find that, in the SM, direct CP asymmetries in thepi+ pi- or K+ K- modes (or in their difference) of the order of several permille are still plausible. Due to the constraints on the new CP phases in the4G model, only moderate effects compared to the SM estimates are possible. Wesuggest CP studies at LHCb as well as at (Super)B-factories of severaldistinctive modes, such as D+ -> Kbar(*)0 pi+, phi pi+ and Ds -> K(*)0 pi+, phipi+ (K+) etc., which should shed more light on the short- and long-distanceissues underlying CP violation in non-leptonic D-meson decays.

Repercussions of Flavour Symmetry Breaking on CP Violation in D-Meson Decays