  The Majorana Demonstrator neutrinoless double-beta decay experiment comprises
a 44 kg (30 kg enriched in $^{76}\mathrm{Ge}$) array of $p$-type, point-contact
germanium detectors. With its unprecedented energy resolution and ultralow
backgrounds, Majorana also searches for rare event signatures from beyond
standard model physics in the low energy region below 100 keV. In this Letter,
we test the continuous spontaneous localization (CSL) model, one of the
mathematically well-motivated wave function collapse models aimed at solving
the long-standing unresolved quantum mechanical measurement problem. While the
CSL predicts the existence of a detectable radiation signature in the x-ray
domain, we find no evidence of such radiation in the 19--100 keV range in a
37.5 kg-y enriched germanium exposure collected between December 31, 2015, and
November 27, 2019, with the Demonstrator. We explored both the
non-mass-proportional (n-m-p) and the mass-proportional (m-p) versions of the
CSL with two different assumptions: that only the quasifree electrons can emit
the x-ray radiation and that the nucleus can coherently emit an amplified
radiation. In all cases, we set the most stringent upper limit to date for the
white CSL model on the collapse rate, $\lambda$, providing a factor of 40--100
improvement in sensitivity over comparable searches. Our limit is the most
stringent for large parts of the allowed parameter space. If the result is
interpreted in terms of the Di\`osi-Penrose gravitational wave function
collapse model, the lower bound with a 95% confidence level is almost an order
of magnitude improvement over the previous best limit.

Nuclear Experiment;High Energy Physics - Experiment