Recent measurements of the charge-parity asymmetry in the decay $D^0\to K_S^0{K}_S^0$ by CMS Collaboration, $A_{CP}(K_S^0{K}_S^0)=(6.2\pm 3.0\pm 0.2\pm 0.8)\%$, and by LHCb, $A_{CP}(D^0\to K_S^0{K}_S^0)=(1.86\pm 1.04\pm 0.41)\%$, suggest possible deviations from Standard Model expectations, which predict asymmetries below the percent level. This singly Cabibbo-suppressed decay is particularly sensitive to new physics, as the leading amplitudes vanish in the exact U-spin symmetry limit and the process is dominated by W-exchange topologies. We investigate scalar diquark contributions to this decay, comparing color-sextet and color-triplet representations. We find that the color-sextet diquark, characterized by a symmetric color structure $(C_1^{\mathrm{NP}}=C_2^{\mathrm{NP}})$, avoids color suppression and can generate $CP$ asymmetries in the range 0.5%–1.5% for a diquark mass of order 1 TeV. In contrast, the color-triplet contribution is strongly suppressed due to destructive interference from its antisymmetric color structure. We further show that a flavor hierarchy in the sextet couplings, with ${\lambda }_{ud}>{\lambda }_{us}$, can simultaneously account for the observed deviation from the U-spin sum rule in $D^0\to K^{+}{K}^{-}$ and $D^0\to {\pi }^{+}{\pi }^{-}$ and the measured $CP$ asymmetry in $D^0\to K_S^0{K}_S^0$. These results identify color-sextet scalar diquarks as viable candidates for explaining enhanced $CP$ violation in charm decays.

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• Extensions of scalar sector
• Particle decays
• Phenomenology
• Charmed mesons
• CP violation