Observation of the Decay D^0 → K^−π^+e^+e^−

We report the observation of the rare charm decay D^0 → K^−π^+e^+e^−, based on 468 fb^(−1) of e^+e^− annihilation data collected at or close to the center-of-mass energy of the Υ(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^−) < 0.875 GeV/c^2 of the electron-positron pair to be B(D^0 → K^−π^+e^+e^−) = (4.0±0.5±0.2±0.1)×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 → K^−π^+π^+π^− 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 → K^−π^+μ^+μ^− 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 B(D^0 → K^−π^+e^+e^−) < 3.1×10^(−6)