Exotic Dark Matter Search with the Majorana Demonstrator

With excellent energy resolution and ultralow-level radiogenic backgrounds, the high-purity germanium detectors in the Majorana Demonstrator enable searches for several classes of exotic dark matter (DM) models. In this work, we report new experimental limits on keV-scale sterile neutrino DM via the transition magnetic moment from conversion to active neutrinos ${\nu }_s\to {\nu }_a$. We report new limits on fermionic dark matter absorption ($\chi +A\to \nu +A$) and sub-GeV DM-nucleus $3\to 2$ scattering ($\chi +\chi +A\to \varphi +A$), and new exclusion limits for bosonic dark matter (axionlike particles and dark photons). These searches utilize the (1–100)-keV low-energy region of a 37.5-kg y exposure collected by the Demonstrator between May 2016 and November 2019 using a set of ${}^{76}\mathrm{Ge}$-enriched detectors whose surface exposure time was carefully controlled, resulting in extremely low levels of cosmogenic activation.