Probing Sub-MeV Dark Matter with Neutron-Capture $γ$ Spectroscopy

Abstract: We present a general, discovery-grade framework for searching for weakly coupled new particles emitted in nuclear de-excitation following neutron capture. Rather than relying on isolated spectral features, the method exploits correlated ``satellite-line combs'': multiple weak $γ$-ray lines appearing at a common energy offset $Δ$ below known capture transitions. By combining likelihood information across many parent lines and multiple target nuclei, the approach strongly suppresses nuclear-structure ambiguities and instrumental artifacts. We also discuss optimal target selection and practical experimental implementation with high-resolution HPGe detectors.