Observation of a Significant Excess ofπ0π0Events inBMeson Decays

We present a study of the decay $B^0\to {\pi }^0{\pi }^0$ based on a sample of $124\times {10}^6$ $B\overline{B}$ pairs recorded by the BABAR detector at the PEP-II asymmetric-energy $B$ Factory at SLAC. We observe $46\pm 13\pm 3$ events, where the first error is statistical and the second is systematic, corresponding to a significance of $4.2$ standard deviations including systematic uncertainties. We measure the branching fraction $\mathcal{B}(B^0\to {\pi }^0{\pi }^0)=(2.1\pm 0.6\pm 0.3)\times {10}^{-6}$, averaged over $B^0$ and ${\overline{B}}^0$ decays.