First Direct Search for Light Dark Matter Using the NEON Experiment at a Nuclear Reactor

We report new results from the neutrino elastic scattering observation with NaI (NEON) experiment in the search for light dark matter (LDM) using 2636  kg·days of NaI(Tl) exposure. The experiment employs an array of NaI(Tl) crystals with a total mass of 16.7 kg, located 23.7 m away from a 2.8 GW thermal power nuclear reactor. We investigated LDM produced by the invisible decay of dark photons, a well-motivated mechanism generated by high-flux photons during reactor operation. The energy spectra collected during reactor-on and reactor-off periods were compared within the LDM signal region of 1-10 keV. No signal consistent with LDM interaction with electrons was observed, allowing us to set 90% confidence level exclusion limits on the dark matter-electron scattering cross section (σ_{e}) across dark matter masses ranging from 1 to 1000  keV/c^{2}. Our results set a 90% confidence level upper limit of σ_{e}=3.17×10^{-35}  cm^{2} for a dark matter mass of 100  keV/c^{2}, marking the best laboratory result in this mass range. Additionally, our search extends the coverage of LDM below 100  keV/c^{2} for the first time, assuming the specific invisible decay of dark photons.