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

TL;DR

This paper identifies specific charm meson decay channels as promising for discovering CP violation within the Standard Model, highlighting advantages in measurement efficiency and potential asymmetry size.

Contribution

It proposes using untagged CP asymmetries in $D^0$ decays to $K_S K^{*0}$ as effective discovery channels, emphasizing their experimental advantages and theoretical significance.

Findings

Untagged CP asymmetry can reach about 0.003.

Decay channels offer experimental efficiency due to prompt $K^{*0}$ decay.

CP asymmetry mainly arises from exchange diagrams, not penguin amplitudes.

Abstract

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.