Observation ofB0→π0π0

We report the observation of the decay $B^0\to {\pi }^0{\pi }^0$, using a $253{\mathrm{fb}}^{-1}$ data sample collected at the ${\rm Y}(4\mathrm{S})$ resonance with the Belle detector at the KEKB $e^{+}{e}^{-}$ collider. The measured branching fraction is $\mathcal{B}(B^0\to {\pi }^0{\pi }^0)=({2.3}_{-0.5-0.3}^{+0.4+0.2})\times {10}^{-6}$, with a significance of 5.8 standard deviations including systematic uncertainties. We also make a measurement of the direct $CP$ violating asymmetry in this mode.