We measure the time-integrated $CP$ asymmetry in $D^0 \to K^0_{\rm S}
K^0_{\rm S}$ decays reconstructed in $e^+e^-\to c{\overline c}$ events
collected by the Belle and Belle II experiments. The corresponding data samples
have integrated luminosities of 980 E 428 fb${}^{-1}$, respectively. To infer
the flavor of the $D^0$ meson, we exploit the correlation between the flavor of
the reconstructed decay and the electric charges of particles reconstructed in
the rest of the $e^+e^-\to c{\overline c}$ event. This results in a sample
which is independent from any other previously used at Belle or Belle II. The
result, $A_{CP}(D^0 \to K^0_{\rm S} K^0_{\rm S}) = (1.3 \pm 2.0 \pm 0.2)\%$,
where the first uncertainty is statistical and the second systematic, is
consistent with previous determinations and with $CP$ symmetry.

Questo articolo esplora i giri e le loro implicazioni.

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