We explore the indirect detection of sterile neutrino dark matter within the gauged U(1) 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 $\textit{N}$ → $\textit{νγ}$, which produces a monochromatic photon, and the three-body decay $\textit{N}$ → $\textit{e}$ $\textit{e}$ $\textit{ν}$, 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 $$\mathcal{O}(100)$$ 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 $\textit{N}$ → $\textit{e}$ $\textit{e}$ $\textit{ν}$, 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.

Japan

Subaru Fujisawa

Tatsuya Hayashi

Shigeki Matsumoto

Yuki Watanabe

Detecting sterile neutrino dark matter at MeV gamma-ray observatories

A non-negligible fraction of the QCD axion dark matter may form gravitationally bound Bose Einstein condensates, which are commonly known as axion stars or axion clumps. Such astrophysical objects have been recently proposed as the cause for the single candidate event reported by Subaru Hyper Suprime-Cam (HSC) microlensing search in the Andromeda galaxy. Depending on the breaking scale of the Peccei-Quinn symmetry and the details of the dark matter scenario, QCD axion clumps may form via gravitational condensation during radiation domination, in the dense core of axion miniclusters, or within axion minihalos around primordial black holes. We analyze all these scenarios and conclude that the microlensing candidate detected by the Subaru HSC survey is likely not caused by QCD axion stars.