Dark matter interpretation of excesses in multiple direct detection experiments

Abstract

We present a novel unifying interpretation of excess event rates observed in several dark matter directdetection experiments that utilize single-electron threshold semiconductor detectors. Despite their different locations, exposures, readout techniques, detector composition, and operating depths, these experiments all observe statistically significant excess event rates of ∼10 Hz=kg. However, none of these persistent excesses has yet been reported as a dark matter signal because individually, each can be attributed to different well-motivated but unmodeled backgrounds, and taken together, they cannot be explained by dark matter particles scattering elastically off detector nuclei or electrons. We show that these results can be reconciled if the semiconductor detectors are seeing a collective inelastic process, consistent with exciting a plasmon. We further show that plasmon excitation could arise in two compelling dark matter scenarios, both of which can explain rates of existing signal excesses in germanium and, at least at the order of magnitude level, across several single-electron threshold detectors. At least one of these scenarios also yields the correct relic density from thermal freeze-out. Both dark matter scenarios motivate a radical rethinking of the standard interpretations of dark matter-electron scattering from recent experiments.

Ionization yield in silicon for eV-scale electron-recoil processes

“…We show that systematics on the order of 50% can arise when applying ionization models in scientific applications due to both the finite band-gap and complex features of the band-structure of crystalline Si which are not averaged out, and that the width of the hole band, rather than plasmon interactions, has a visible impact on the charge yield in the regime between 12-50 eV. Disentangling the effect of ionization is vital for correctly attributing the response of the detector to the physics of dark matter or some other unmeasured process, such as potential signals from elastic nuclear recoil, the Migdal effect1 [11], or other collective effects [12,13].…”

Plasmon production from dark matter scattering

“…(1) is typically 4-5 orders of magnitude smaller than the elastic nuclear recoil rate, and therefore cannot explain the excesses studied in Ref. [21]. (Note that the mechanism of Ref.…”

“…(Note that the mechanism of Ref. [21] involved a plasmon produced in association with many phonons, and is therefore not captured by our approach.) Nevertheless, bremsstrahlung emission of plasmons by a recoiling nucleus is a novel signature of dark matter scattering in semiconductor targets, and we find that the corresponding rate is around 5 orders of magnitude larger than that for bremsstrahlung emission of transverse modes.…”

“…Recently, Ref. [21] has highlighted a number of lowenergy residual rates in experiments achieving the lowest thresholds thus far, and points out the relevance of manybody effects for understanding them. The rates are comparable in SENSEI [22], CDMS HVeV [23], and EDELWEISS [24,25], though much lower in DAMIC [26].…”

“…Reference [21] has proposed that such excesses could be explained as DM exciting plasmons in semiconductors, since no excesses with corresponding rates have been observed in noble liquid experiments such as XENON1T [27] and DarkSide [28]. One of their proposed ideas is the secondary production of plasmons during DM-nucleus scattering from DM with mass in the 30 MeV-GeV range.…”