Observation of the Decay $D^0\to K^{-}{\pi }^{+}{e}^{+}{e}^{-}$

We report the observation of the rare charm decay $D^0\to K^{-}{\pi }^{+}{e}^{+}{e}^{-}$, based on $468{\mathrm{fb}}^{-1}$ of $e^{+}{e}^{-}$ annihilation data collected at or close to the center-of-mass energy of the $\mathrm{\Upsilon }(4\mathrm{S})$ resonance with the BABAR detector at the SLAC National Accelerator Laboratory. We find the branching fraction in the invariant mass range $0.675<m(e^{+}{e}^{-})<0.875\mathrm{GeV}/c^2$ of the electron-positron pair to be $\mathcal{B}(D^0\to K^{-}{\pi }^{+}{e}^{+}{e}^{-})=(4.0\pm 0.5\pm 0.2\pm 0.1)\times 10^{-6}$, where the first uncertainty is statistical, the second systematic, and the third due to the uncertainty in the branching fraction of the decay $D^0\to K^{-}{\pi }^{+}{\pi }^{+}{\pi }^{-}$ used as a normalization mode. The significance of the observation corresponds to 9.7 standard deviations including systematic uncertainties. This result is consistent with the recently reported $D^0\to K^{-}{\pi }^{+}{\mu }^{+}{\mu }^{-}$ branching fraction, measured in the same invariant mass range, and with the value expected in the standard model. In a set of regions of $m(e^{+}{e}^{-})$, where long-distance effects are potentially small, we determine a 90% confidence level upper limit on the branching fraction $\mathcal{B}(D^0\to K^{-}{\pi }^{+}{e}^{+}{e}^{-})<3.1\times 10^{-6}$.

Figure 1

Fits to $D^0\to K^{-}{\pi }^{+}{e}^{+}{e}^{-}$ data distributions for (a) $m(K^{-}{\pi }^{+}{e}^{+}{e}^{-})$ and (b) $\mathrm{\Delta }m$ mass for candidates with $0.675<m(e^{+}{e}^{-})<0.875\mathrm{GeV}/c^2$.

Figure 2

Projections of the fits to the $D^0\to K^{-}{\pi }^{+}{e}^{+}{e}^{-}$ data distributions onto (a) $m(e^{+}{e}^{-})$ and (b) $m(K^{-}{\pi }^{+})$ for candidates with $0.675<m(e^{+}{e}^{-})<0.875\mathrm{GeV}/c^2$. The background has been subtracted using the sPlot technique [37].

Figure 3

Projection of the fits to the $D^0\to K^{-}{\pi }^{+}{e}^{+}{e}^{-}$ data distributions onto $m(e^{+}{e}^{-})$ for candidates with $m(e^{+}{e}^{-})>0.2\mathrm{GeV}/c^2$. The background has been subtracted using the sPlot technique [37]. The shaded bands indicate the $m(e^{+}{e}^{-})$ regions excluded from the “continuum” region.