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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 → Kπ+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 → Kπ+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 → Kπ+e+e-) < 3.1 × 10−6.

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Lees, J. P., Poireau, V., Tisserand, V., Grauges, E., Palano, A., Eigen, G., Brown, D. N., Kolomensky, Yu. G., Fritsch, M., Koch, H., Schroeder, T., Hearty, C., Mattison, T. S., McKenna, J. A., So, R. Y., Blinov, V. E., Buzykaev, A. R., Druzhinin, V. P., Golubev, V. B., … Wu, S. L. (2019a). Observation of the decay . Physical Review Letters, 122(8).

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