Search for Rare or Forbidden Decays of the $D^0$ Meson

We present a search for nine lepton-number-violating and three lepton-flavor-violating neutral charm decays of the type $D^0\to h^{\prime -}{h}^{-}{\ell }^{\prime +}{\ell }^{+}$ and $D^0\to h^{\prime -}{h}^{+}{\ell }^{\prime \pm }{\ell }^{\mp }$, where $h$ and $h^{\prime }$ represent a $K$ or $\pi$ meson and $\ell$ and ${\ell }^{\prime }$ an electron or muon. The analysis is based on $468{\mathrm{fb}}^{-1}$ of $e^{+}{e}^{-}$ annihilation data collected at or close to the $\mathrm{\Upsilon }(4S)$ resonance with the BABAR detector at the SLAC National Accelerator Laboratory. No significant signal is observed for any of the twelve modes, and we establish 90% confidence level upper limits on the branching fractions in the range $(1.0–30.6)\times {10}^{-7}$. The limits are between 1 and 3 orders of magnitude more stringent than previous measurements.

Figure 1

Projections of the unbinned maximum-likelihood fits to the final candidate distributions as a function of $\mathrm{\Delta }m$ for the signal modes with fewer than two kaons. The solid blue line is the total fit, the dashed red line is the signal, and the dotted green line is the background.

Figure 2

Projections of the unbinned maximum-likelihood fits to the final candidate distributions as a function of $\mathrm{\Delta }m$ for the signal modes with two kaons. The solid blue line is the total fit, the dashed red line is the signal, and the dotted green line is the background.