PNNL

Abstract

The Majorana Demonstrator was a neutrinoless double-beta decay search experiment comprised of 44 kg (30 kg enriched in 76Ge) array of p-type, point-contact germanium detectors. With its unprecedented energy resolution and ultra-low backgrounds, Majorana also searched for rare event signatures from beyond Standard Model physics in the low energy region below keV. In this letter, we test the continuous spontaneous localization (CSL) model, one of the mathematically well-motivated wavefunction 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 5-46 keV range in a 38.35 kg-y enriched germanium exposure collected between Dec. 31, 2015 and Nov. 27, 2018 with the Demonstrator. We explored both the mass-proportional (m-p) and the non-mass-proportional (nm-p) versions of the CSL with two different assumptions: that only the quasi-free 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 on the collapse rate, , in the parameter space below rC = 10??6 m for the white CSL model, where rC is the correlation length of the collapse noise field. If the result is interpreted in terms of the Diòsi-Penrose (DP) gravitational wavefunction collapse model, the 95% lower bound is more than two orders of magnitude improvement over the previous best limit.