Neutral $D\rightarrow K K^*$ decays as discovery channels for charm CP violation

We point out that the CP asymmetries in the decays $D^0\rightarrow K_{S} K^{*0}$ and $D^0\rightarrow K_{S} \overline{K}{}^{*0}$ are potential discovery channels for charm CP violation in the Standard Model. We stress that no flavor tagging is necessary, the untagged CP asymmetry $a_{CP}^{\mathrm{dir}}(D \to K_S K^{*0})$ is essentially equal to the tagged one, so that the untagged measurement comes with a significant statistical gain. Depending on the relevant strong phase, $\vert a_{CP}^{\text{dir, untag}}\vert$ can be as large as $0.003$. The CP asymmetry is dominantly generated by exchange diagrams and does not require non-vanishing penguin amplitudes. While the CP asymmetry is smaller than in the case of $D^0\rightarrow K_SK_S$, the experimental analysis is more efficient due to the prompt decay $K^{*0}\rightarrow K^+\pi^-$. One may further search for favourable strong phases in the Dalitz plot in the vicinity of the $K^{*0}$ peak.