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

Abstract

We report new results from the Neutrino Elastic Scattering Observation with NaI (NEON) experiment in the search for light dark matter (LDM) using 2,636 kg$\cdot$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 meters away from a 2.8 GW thermal power nuclear reactor. We investigated LDM produced by the $\text{invisible decay}$ of dark photons 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 for the dark matter-electron scattering cross-section ($\ddot{I}{ƒ}_e$) across dark matter masses ranging from 1 keV/c${}^2$ to 1 MeV/c${}^2$. Our results set a 90% confidence level upper limit of $\ddot{I}{ƒ}_e=3.17\times 10^{-35}\mathrm{c}{\mathrm{m}}^2$ 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${}^2$ first time.