Detecting Sterile Neutrino Dark Matter at MeV Gamma-Ray Observatories

We explore the indirect detection of sterile neutrino dark matter within the gauged $U(1)_{B-L}$ extension of the Standard Model, in which three right-handed neutrinos account for neutrino masses, the baryon asymmetry, and dark matter. Focusing on the MeV mass range, we investigate two decay channels: the radiative decay $N \to \nu \gamma$, which produces a monochromatic photon, and the three-body decay $N \to e^- e^+ \nu$, which leads to a 511 keV photon signal from positronium decay. Taking the upcoming COSI mission as a case study, we show that both signals are experimentally accessible and complementary, with the 511 keV channel extending the sensitivity reach up to $O(100)$ MeV. We propose a novel analysis strategy in Compton data space to isolate the diffuse 511 keV emission. Furthermore, we incorporate, for the first time, the Sommerfeld enhancement in the decay width of $N \to e^- e^+ \nu$, enabling more accurate predictions of the signal near the kinematic threshold. The combined observation of both channels would provide a distinctive and testable signature of the sterile neutrino dark matter hypothesis.