New Constraints on Axion-Like Particles with the NEON Detector at a Nuclear Reactor

Abstract: We report new constraints on axion-like particles (ALPs) using data from the NEON experiment, which features a 16.7 kg of NaI(Tl) target located 23.7 meters from a 2.8 GW thermal power nuclear reactor. Analyzing a total exposure of 3063 kg$\cdot$days, with 1596 kg$\cdot$days during reactor-on and 1467 kg$\cdot$days during reactor-off periods, we compared energy spectra to search for ALP-induced signals. No significant signal was observed, enabling us to set exclusion limits at the 95\% confidence level. These limits probe previously unexplored regions of the ALP parameter space, particularly for axion mass ($m_a$) near $1$ MeV/c$^2$. For ALP-photon coupling (${g_{a\gamma}}$), limits reach as low as 6.24$\times$ 10$^{-6}$ GeV$^{-1}$ at $m_a$ = 3.0 MeV/c$^2$, while for ALP-electron coupling (${g_{ae}}$), limits reach 4.95$\times$ 10$^{-8}$ at $m_a$ = 1.02 MeV/c$^2$. This work demonstrates the potential for future reactor experiments to probe unexplored ALP parameter space.