First search for direct $CP$-violating asymmetry in $B^0\to K^0\ddot{I}{€}^0$ decays at Belle II

Abstract

We report on the first measurement of the direct $CP$-violating asymmetry ($\mathcal{A}$) in the charmless decay $B^0\to K^0\ddot{I}{€}^0$ at Belle II and an updated measurement of its branching fraction ($\mathcal{B}$). We use a sample of electron-positron collisions collected in 2019 and 2020 at the $\hat{I}\yen (4S)$ resonance and corresponding to $62.8$ ${\text{fb}}^{-1}$ of integrated luminosity. We reconstruct and select about $50$ $B^0\to K_S^0\ddot{I}{€}^0$ candidates, and we measure ${\mathcal{A}}_{K^0\ddot{I}{€}^0}=-0.40_{-0.44}^{+0.46}(\text{stat})\pm 0.04(\text{syst})$ and $\mathcal{B}(B^0\to K^0\ddot{I}{€}^0)=[8.5_{-1.6}^{+1.7}(\text{stat})\pm 1.2(\text{syst})]\times 10^{-6}$. This is the first measurement of $CP$ violation in $B^0\to K^0\ddot{I}{€}^0$ decays reported by Belle II. The results agree with previous determinations and show a detector performance comparable with the best Belle results.

19 pages, 5 figures. Supporting material for Winter 2021 conferences