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.

Figures (2)

• Figure 1: Profile likelihood scan $q(\Delta)$ for the combined neutron-capture targets. The maximum $q$ corresponds to the recovered $\Delta$, indicated by the dashed red line. The black dashed line shows a parabolic fit around the peak to extract $\Delta$ and the associated uncertainty.
• Figure 2: Combined profile-likelihood scan $q(\Delta)$ obtained assuming a detector resolution of $\sigma_{\rm det}\simeq 4~\mathrm{keV}$. For sufficiently small offsets $\Delta$, the satellite line overlaps the smeared low-energy tail of the parent peak, leading to a partial degeneracy between the satellite contribution and the parent normalization.