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

28 February 2019

Abstract: We report the observation of the rare charm decay ${D}^{0}\to {K}^{-}{\pi }^{+}{e}^{+}{e}^{-}$, based on $468\text{}\text{}{\mathrm{fb}}^{-1}$ of ${e}^{+}{e}^{-}$ annihilation data collected at or close to the center-of-mass energy of the $\Upsilon \left(4S\right)$ resonance with the BABAR detector at the SLAC National Accelerator Laboratory. We find the branching fraction in the invariant mass range $0.675 of the electron-positron pair to be $B\left({D}^{0}\to {K}^{-}{\pi }^{+}{e}^{+}{e}^{-}\right)=\phantom{\rule{0ex}{0ex}}\left(4.0±0.5±0.2±0.1\right)×1{0}^{-6}$, 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 $m\left({e}^{+}{e}^{-}\right)$, where long-distance effects are potentially small, we determine a 90% confidence level upper limit on the branching fraction $B\left({D}^{0}\to {K}^{-}{\pi }^{+}{e}^{+}{e}^{-}\right)<3.1×1{0}^{-6}$.

Published in: Physical Review Letters 122 (2019)