Measurement of the branching fraction and $CP$ asymmetry of $B^0\to {\pi }^0{\pi }^0$ decays using $198\times {10}^6$ $B\bar{B}$ pairs in Belle II data

We report measurements of the branching fraction and $CP$ asymmetry in $B^0\to {\pi }^0{\pi }^0$ decays reconstructed at Belle II in an electron-positron collision sample containing $198\times {10}^6$ $B\bar{B}$ pairs. We measure a branching fraction $\mathcal{B}(\text{Bpipi})=(1.38\pm 0.27\pm 0.22)\times {10}^{-6}$ and a $CP$ asymmetry $\text{Acp}(\text{Bpipi})=0.14\pm 0.46\pm 0.07$, where the first uncertainty is statistical and the second is systematic.