Observation of the decay D⁰→K−π+e+e−

Abstract

We report the observation of the rare charm decay D[superscript 0]→K[superscript -]π[superscript +]e[superscript +]e[superscript -], based on 468 fb-¹ of e[superscript +]e[superscript -] 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[superscript +]e[superscript -])<0.875 GeV/c² of the electron-positron pair to be B(D⁰→K[superscript -]π[superscript +]e[superscript +]e[superscript -])=(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[superscript -]π[superscript +]π[superscript +]π[superscript -] 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[superscript -]π[superscript +]μ[superscript +]μ[superscript -] 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[superscript +]e[superscript -]), where long-distance effects are potentially small, we determine a 90% confidence level upper limit on the branching fraction B(D⁰→K[superscript -]π[superscript +]e[superscript +]e[superscript -])<3.1×10-⁶. ©2018 keywords: leptonic, semileptonic & radiative decays; charmed mesons; lepton colliders