Observation of a Significant Excess of ${\pi }^0{\pi }^0$ Events in $B$ Meson 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.

Figure 1

The distributions of (a) $m_{\mathrm{E}\mathrm{S}}$, (b) $\Delta E$, and (c) Fisher discriminant $F$ for candidates in the signal data sample, and (d) the $F$ distribution for candidates in the sideband data sample. The solid lines show the PDF for signal plus background. For $m_{\mathrm{E}\mathrm{S}}$ and $\Delta E$, the dashed lines show the PDF for the $q\overline{q}$ background. The abscissa in (c) and (d) is the $F$ bin number, where the bins have been chosen so that each bin contains approximately 10% of the distribution for the $B^0\to D^{(*)}n\pi$ data sample.

Figure 2

The distributions of (a) $m_{\mathrm{E}\mathrm{S}}$, (b) $\Delta E$, and (c) Fisher discriminant $F$ for candidates in the signal data sample that satisfy an optimized requirement on the signal probability, based on all variables except the one being plotted. The fraction of signal events included in the plots is 24%, 42%, and 74% for $m_{\mathrm{E}\mathrm{S}}$, $\Delta E$, and $F$, respectively. The PDF projections are shown as a dashed line for $q\overline{q}$ background, a dotted line for $B^{\pm }\to {\rho }^{\pm }{\pi }^0$, and a dash-dotted line for $B^0\to {\pi }^0{\pi }^0$ signal. The solid line shows the sum of all PDF projections. The PDF projections are scaled by the expected fraction of events passing the probability-ratio requirement. The ratio $-2\ln (\mathcal{L}/{\mathcal{L}}_{\mathrm{m}\mathrm{a}\mathrm{x}})$ is shown in (d) where the dashed line corresponds to statistical errors only and the solid line corresponds to statistical and systematic errors, as applied for the calculation of significance.