The Majorana Demonstrator is an ultralow-background experiment searching for neutrinoless double-beta decay in ${}^{76}\mathrm{Ge}$. The heavily shielded array of germanium detectors, placed nearly a mile underground at the Sanford Underground Research Facility in Lead, South Dakota, also allows searches for new exotic physics. Free, relativistic, lightly ionizing particles with an electrical charge less than $e$ are forbidden by the standard model but predicted by some of its extensions. If such particles exist, they might be detected in the Majorana Demonstrator by searching for multiple-detector events with individual-detector energy depositions down to 1 keV. This search is background-free, and no candidate events have been found in 285 days of data taking. New direct-detection limits are set for the flux of lightly ionizing particles for charges as low as $e/1000$.

• Received 30 January 2018
• Revised 23 April 2018

DOI:https://doi.org/10.1103/PhysRevLett.120.211804

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Published by the American Physical Society