# Search for $CP$ Violation and Measurement of the Branching Fraction in the Decay $D^{0} \to K^0_S K^0_S$

## TL;DR

This paper measures the $CP$ asymmetry and branching fraction of the decay $D^0 o K^0_S K^0_S$ using a large data sample, providing more precise results consistent with the standard model.

## Contribution

It presents the most precise measurement to date of $CP$ violation and branching fraction for $D^0 o K^0_S K^0_S$ decay, using 921 fb$^{-1}$ of data from Belle.

## Key findings

- $A_{CP}$ is consistent with zero within uncertainties.
- Branching fraction measured as (1.321 ± 0.023 ± 0.036 ± 0.044) × 10$^{-4}$.
- Results are more precise than previous measurements.

## Abstract

We report a study of the decay $D^0 \to K^0_S K^0_S$ using 921~fb$^{-1}$ of data collected at or near the $\Upsilon(4S)$ and $\Upsilon(5S)$ resonances with the Belle detector at the KEKB asymmetric energy $e^+e^-$ collider. The measured time-integrated $CP$ asymmetry is $ A_{CP}(D^0 \to K^0_S K^0_S) = (-0.02 \pm 1.53 \pm 0.02 \pm 0.17) \%$, and the branching fraction is $\mathcal{B} (D^{0}\rightarrow K_{S}^{0}K_{S}^{0})$ = (1.321 $\pm$ 0.023 $\pm$ 0.036 $\pm$ 0.044) $\times$ 10$^{-4}$, where the first uncertainty is statistical, the second is systematic, and the third is due to the normalization mode ($D^0 \to K_S^0 \pi^0$). These results are significantly more precise than previous measurements available for this mode. The $A_{CP}$ measurement is consistent with the standard model expectation.