Measurement of the branching fraction and $CP$ asymmetry in $B^0\to {\pi }^0{\pi }^0$ decays, and an improved constraint on ${\phi }_2$

Here, we measure the branching fraction and $CP$ violation asymmetry in the decay $B^0\to {\pi }^0{\pi }^0$, using a data sample of $752\times {10}^{6}B\overline{B}$ pairs collected at the ${\rm Y}\left(4S\right)$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. The obtained branching fraction and direct $CP$ asymmetry are $B(B\to {\pi }^0{\pi }^0)=[1.31\pm 0.19\left(\mathrm{stat}\right)\pm 0.19\left(\mathrm{syst}\right)]\times {10}^{-6}$ and $A_{CP}=+0.14\pm 0.36\left(\mathrm{stat}\right)\pm 0.10\left(\mathrm{syst}\right)$, respectively. The signal significance, including the systematic uncertainty, is 6.4 standard deviations. Furthemore, we combine these results with Belle's earlier measurements of $B^0\to {\pi }^{+}{\pi }^{-}$ and $B^{\pm }\to {\pi }^{\pm }{\pi }^0$ to exclude the $CP$-violating parameter ${\phi }_2$ from the range $15.5°<{\phi }_2<75.0°$ at 95% confidence level.