Search for $D^0\to \gamma \gamma$ and improved measurement of the branching fraction for $D^0\to {\pi }^0{\pi }^0$

Using $2.92{\mathrm{fb}}^{-1}$ of electron-positron annihilation data collected at $\sqrt{s}=3.773\mathrm{GeV}$ with the BESIII detector, we report the results of a search for the flavor-changing neutral current process $D^0\to \gamma \gamma$ using a double-tag technique. We find no signal and set an upper limit at 90% confidence level for the branching fraction of $\mathcal{B}(D^0\to \gamma \gamma )<3.8\times {10}^{-6}$. We also investigate $D^0$-meson decay into two neutral pions, obtaining a branching fraction of $\mathcal{B}(D^0\to {\pi }^0{\pi }^0)=(8.24\pm 0.21(\text{stat})\pm 0.30(\text{syst}))\times {10}^{-4}$, the most precise measurement to date and consistent with the current world average.

Figure 1

Fits (solid line) to the $M_{\mathrm{BC}}^{\mathrm{tag}}$ distributions in data (points) for the five ${\overline{D}}^0$ tag modes: (a) $K^{+}{\pi }^{-}$, (b) $K^{+}{\pi }^{-}{\pi }^0$, (c) $K^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-}$, (d) $K^{+}{\pi }^{-}{\pi }^{+}{\pi }^{-}{\pi }^0$, and (e) $K^{+}{\pi }^{-}{\pi }^0{\pi }^0$. The gray shaded histograms are arbitrarily scaled generic MC backgrounds.

Figure 2

Fit to the DT sample in data (points), projected onto $\mathrm{\Delta }{E}^{\gamma \gamma }$ (a) and $\mathrm{\Delta }{E}^{\mathrm{tag}}$ (b). The dashed lines show the overall fits, while the dotted histograms represent the estimated background contribution from $D^0\to {\pi }^0{\pi }^0$. The solid line superimposed on the $\mathrm{\Delta }{E}^{\gamma \gamma }$ projection indicates the expected signal for $\mathcal{B}(D^0\to \gamma \gamma )=10\times {10}^{-6}$. Also overlaid are the overall MC-estimated backgrounds (gray shaded histograms) and the background component from non-$D\overline{D}$ processes (diagonally hatched histograms).

Figure 3

Fit to the $M_{\mathrm{BC}}^{{\pi }^0{\pi }^0}$ distribution in data (points) for $D^0\to {\pi }^0{\pi }^0$ DT candidates. The solid line is the total fitted result, while the dotted and dashed lines are the background and signal components of the fit, respectively. The diagonally shaded histogram is the background determined with MC.

Figure 4

Accumulated branching fraction distribution based on toy MC samples generated from the data-driven PDF. (See the text for details.) The shaded region represents 90% of the physical region.

Figure 5

Fit to the $M_{\mathrm{BC}}^{{\pi }^0{\pi }^0}$ distribution in data for $D^0\to {\pi }^0{\pi }^0$ candidates (points). The shaded histogram is the background predicted by MC. The solid and dashed curves are the total fit and the background component, respectively, and the dotted curve shows the signal.