Observation of the decay $D^0\to K^{-}\ddot{I}{€}^{+}{e}^{+}{e}^{-}$

Abstract

We report the observation of the rare charm decay $D^0\to K^{-}\ddot{I}{€}^{+}{e}^{+}{e}^{-}$, based on $468$ fb${}^{-1}$ of $e^{+}{e}^{-}$ annihilation data collected at or close to the center-of-mass energy of the $Y(4S)$ resonance with the BaBar detector at the SLAC National Accelerator Laboratory. We find the branching fraction in the invariant mass range $0.675<m(e^{+}{e}^{-})<0.875GeV/c^2$ of the electron-positron pair to be $\mathcal{B}(D^0\to K^{-}\ddot{I}{€}^{+}{e}^{+}{e}^{-})=(4.0\pm 0.5\pm 0.2\pm 0.1)\times 10^{-6}$, where the first uncertainty is statistical, the second systematic, and the third due to the uncertainty in the branching fraction of the decay $D^0\to K^{-}\ddot{I}{€}^{+}\ddot{I}{€}^{+}\ddot{I}{€}^{-}$ 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 $D^0\to K^{-}\ddot{I}{€}^{+}\hat{I}{¼}^{+}\hat{I}{¼}^{-}$ 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 $\mathcal{B}(D^0\to K^{-}\ddot{I}{€}^{+}{e}^{+}{e}^{-})<3.1\times 10^{-6}$.

7 pages, 3 figures