Reactor-based Search for Axion-Like Particles using CsI(Tl) Detector

The absence of conclusive signals in weakly interacting massive particle (WIMP) searches has motivated increased interest in alternative dark matter candidates such as axions and axion-like particles (ALPs), which also provide a solution to the strong CP problem. In this work, we employ a $\sim 100\mathrm{k}\mathrm{g}$ scale CsI(Tl)-based detector operated in proximity to a nuclear reactor to achieve a sub-100 DRU (differential rate unit, expressed in counts/keV/kg/day) background level in the MeV energy range through a combination of active veto and passive shielding techniques. Such a low-background environment enables sensitivity to ALPs with axion–photon coupling $g_{a\gamma \gamma }\gtrsim {10}^{-6}$ and axion–electron coupling in the range ${10}^{-8}<g_{aee}<{10}^{-4}$ for ALP masses between 1~keV and 10~MeV. These results demonstrate that the experiment has the potential to probe previously unexplored regions of parameter space, including the so-called cosmological triangle in the ALP–photon coupling for MeV-scale ALPs.