Nuclear Reactor Data Excludes Cosmological Triangle in Search for Axion-Like Particles

We report new constraints on axion-like particle (ALP) using data corresponding to a sodium iodine target exposure of 3063\,kg·days from the neutrino elastic scattering observation with NaI (NEON) experiment. A 16.7\,kg of thallium-doped sodium iodide target was located 23.7 meters from a 2.8\,GW thermal power nuclear reactor. We searched for ALPs produced by high-flux photons by comparing the energy spectra of data collected during reactor-on (1596\,kg·days exposure) and reactor-off (1467\,kg·days exposure) periods. No signal consistent with ALP interaction was identified, allowing us to set exclusion limits at the 95% confidence level. Our limits cover previously unexplored regions for both photon couplings (gaγ) and electron couplings (gₐₑ) for axion masses around 1\,MeV/c². Notably, the NEON data excludes the unconstrained region identified by laboratory-based searches for photon couplings within the ``cosmological triangle'' for the first time. The observed 95% confidence level limits reach as low as gaγ of 4.33× 10⁻⁸\,GeV⁻¹ and gₐₑ of 1.10× 10⁻⁹ for axion masses of 1.7\,MeV/c² and 1.0\,MeV/c², respectively.