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

We search for the rare decay of the $D^0$ meson to two photons, $D^0\to \gamma \gamma$, and present a measurement of the branching fraction for a $D^0$ meson decaying to two neutral pions, $B(D^0\to {\pi }^0{\pi }^0)$. The data sample analyzed corresponds to an integrated luminosity of $470.5{\mathrm{fb}}^{-1}$ collected by the BABAR detector at the PEP-II asymmetric-energy $e^{+}{e}^{-}$ collider at SLAC. We place an upper limit on the branching fraction, $B(D^0\to \gamma \gamma )<2.2\times {10}^{-6}$, at 90% confidence level. This limit improves on the existing limit by an order of magnitude. We also find $B(D^0\to {\pi }^0{\pi }^0)=(8.4\pm 0.1\pm 0.4\pm 0.3)\times {10}^{-4}$.

Figure 1

The $\gamma \gamma$ mass distribution for $D^0\to \gamma \gamma$ candidates in data (data points). The curves show the result of an unbinned maximum likelihood fit to the measured mass distribution. The solid blue curve corresponds to signal component resulting in a slight negative yield, the long-dash red curve corresponds to combinatoric background component, and the small-dash pink curve corresponds to the combinatoric background plus $D^0\to {\pi }^0{\pi }^0$ background shape. The ${\chi }^2$ value is determined from binned data and is provided as a goodness-of-fit measure. The pull distribution shows differences between the data and the solid blue curve with values and errors normalized.

Figure 2

The ${\pi }^0{\pi }^0$ mass distribution for $D^0\to {\pi }^0{\pi }^0$ candidates in data (data points). The curves show the result of the unbinned maximum likelihood fit to the measured mass distribution. The solid blue curve corresponds to the full PDF including the signal and the dashed red curve corresponds to the combinatoric background component. The ${\chi }^2$ value is determined from binned data and is provided as a goodness-of-fit measure. The pull distribution shows differences between the data and the solid blue curve with values and errors normalized.

Figure 3

Distribution of branching fraction calculations when varying all sources of uncertainty.