Measurement of the branching fraction and $CP$ asymmetry in $B^{0} to pi^{0}pi^{0}$ decays, and an improved constraint on $phi_{2}$

We measure the branching fraction and $CP$ violation asymmetry in the decay $B^{0}to pi^{0}pi^{0}$, using a data sample of $752times 10^{6}$ $Bbar{B}$ pairs collected at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^{+}e^{-}$ collider. The obtained branching fraction and direct $CP$ asymmetry are $ mathcal{B}(Bto pi^{0}pi^{0}) = [1.31 pm 0.19~ text{(stat.)} pm 0.19~ text{(syst.)}] times 10^{-6}$ and $ A_{CP} = +0.14 pm 0.36~ text{(stat.)} pm 0.10~ text{(syst.)}, $ respectively. The signal significance, including the systematic uncertainty, is 6.4 standard deviations. We combine these results with Belles 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 $15.5^{circ} < phi_{2} < 75.0^{circ}$ at 95% confidence level.