**CP asymmetries in $D$ decays to two pseudoscalars**

*U. Nierste, S. Schacht*

in 9th International Workshop on the CKM Unitarity Triangle

Contribution: pdf

**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.