Observation of the Decay D⁰ → K⁻ π ⁺ e ⁺ e ⁻

Abstract

We report the observation of the rare charm decay D0 → K−πþeþe−, based on 468 fb−1 of eþe− annihilation data collected at or close to the center-of-mass energy of the ϒð4SÞ 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=c2 of the electron-positron pair to be B(D0 → 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 D0 → 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 D0 → 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(D0 → K−π+e+e-) < 3.1 × 10−6.