Measurement of the branching fraction and $CP$-violating asymmetry of the decay $B^0\to {\pi }^0{\pi }^0$ using $387$ million bottom-antibottom meson pairs in Belle II data

We measure the branching fraction and $CP$-violating flavor-dependent rate asymmetry of $B^0\to {\pi }^0{\pi }^0$ decays reconstructed using the Belle II detector in an electron-positron collision sample containing $387\times {10}^6$ $B\bar{B}$ pairs. Using an optimized event selection, we find $126\pm 20$ signal decays in a fit to background-discriminating and flavor-sensitive distributions. The resulting branching fraction is $(1.25\pm 0.23)\times {10}^{-6}$ and the $CP$-violating asymmetry is $0.03\pm 0.30$.