Standard model benchmarks for D⁰ → K⁻K⁺, π⁻π⁺, KS0KS0 decays

The non-leptonic D⁰ → K⁻K⁺ and D⁰ → π⁻π⁺ decays are powerful probes of the Standard Model and are related to each other through the U -spin symmetry of the strong interaction. Using lattice QCD inputs we calculate the corresponding colour-allowed tree amplitudes in factorisation and demonstrate that non-factorisable contributions and U -spin-breaking effects at the level of 50% allow us to accommodate the measured branching ratios in the Standard Model. An exciting direct probe of such non-factorisable and U -spin breaking effects is provided by the D0→KS0KS0 channel. This decay is governed by non-factorisable exchange topologies and essentially vanishes in the U -spin limit, although it is experimentally well established with a prominent branching ratio. Extrapolating our D⁰ → K⁻K⁺ results using the isospin symmetry, we find a consistent benchmark picture. Specifically, we can accommodate the measured D0→KS0KS0 branching ratio with U -spin-breaking effects at the 50% level and exchange amplitudes at the level of 50% of the colour-allowed D⁰ → K⁻K⁺, D⁰ → π⁻π⁺ tree contributions. Finally, we explore the resulting range for direct CP violation in D0→KS0KS0, obtaining upper bounds in our benchmark scenarios of a few per mille, offering an exciting target for future measurements.