  We report a measurement of the time-integrated $CP$ asymmetry in the neutral
charm meson decay $D^0 \to K^0_S K^0_S$ using 921~fb$^{-1}$ data collected at
the $\Upsilon(4S)$ and $\Upsilon(5S)$ resonances with the Belle detector at the
KEKB asymmetric-energy $e^+e^-$ collider. The observed asymmetry is $$
A_{CP}(D^0 \to K^0_S K^0_S) = (-0.02 \pm 1.53 \pm 0.17) \%, $$ where the first
uncertainty is statistical and the second systematic. This latter uncertainty
is dominated by the error of the normalisation channel. The result is
consistent with Standard Model expectations and improves the uncertainty with
respect to previous measurement of this quantity by more than a factor of
three.

High Energy Physics - Experiment