# CP asymmetries in $D\to PP^\prime$ decays

## TL;DR

This paper explores CP asymmetries in D meson decays, proposing sum rules to distinguish new physics from the Standard Model and highlighting the decay D^0 → K_S K_S as a promising channel for observing CP violation.

## Contribution

It introduces sum rules relating CP asymmetries in D decays to differentiate new physics effects from the Standard Model and emphasizes the potential of D^0 → K_S K_S decay to reveal large CP violation.

## Key findings

- Sum rules can distinguish new physics from the Standard Model in D decays.
- CP asymmetry in D^0 → K_S K_S can reach 1.1% within the Standard Model.
- D^0 → K_S K_S is a promising channel for discovering charm CP violation.

## Abstract

This talk addresses two topics related to CP asymmetries in $D$ meson decays to two pseudoscalar mesons. First I discuss how new physics can be distinguished from the Standard Model through two sum rules relating three direct CP asymmetries each, using the sum rule correlating $D^0\to K^+K^-$, $D^0\to \pi^+ \pi^-$, and $D^0\to \pi^0 \pi^0$ for illustration. The other sum rule involves $a_{CP}^{\mathrm{dir}}(D^+\rightarrow \bar{K}^0K^+)$, $a_{CP}^{\mathrm{dir}}(D_s^+\rightarrow K^0\pi^+)$, and $a_{CP}^{\mathrm{dir}}(D_s^+\to K^+ \pi^0)$. The second topic is the direct CP asymmetry in the decay $D^0 \to K_S K_S$, which is expected to be large in the Standard Model for two reasons: Flavor-SU(3) symmetry suppresses the tree amplitude which enhances the crucial "penguin-to-tree" ratio and the "penguin" amplitude is dominated by the tree-level $W$ boson exchange between $c$ and $u$ quarks. We find that $|a_{CP}^{\mathrm{dir}} (D^0 \to K_S K_S)|$ can be as large as $1.1 \%$ in the Standard Model. We advocate $D^0 \to K_S K_S$ as a discovery channel for charm CP violation.