Improved measurement of $B^0 \to \pi^0 \pi^0$

We report an improved measurement of the decay $B^0 \to \pi^0 \pi^0$, using a data sample of 535 $\times 10^6 B\bar{B}$ pairs collected at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric energy $e^+ e^-$ collider. The measured branching fraction is ${\cal B}(B^0 \to \pi^0 \pi^0) = 1.1\pm 0.3(stat.) \pm 0.1(syst.)$, with a significance of 5.4 standard deviations including systematic uncertainties. We also report the partial rate asymmetry: $\acp(B^0 \to \pi^0 \pi^0)$ = 0.44 $^{+0.73}_{-0.62}$(stat.)$^{+0.04}_{-0.06}$(syst.).