Abstract
We present a novel unifying interpretation of excess event rates observed in several dark matter direct-detection 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 $\sim$ 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.
Highlights
Searches for particle dark matter (DM) with masses below 1 GeV have proliferated in the last decade, driven by advances in detector technologies which have pushed heat detection thresholds below 100 eV [1,2] and charge detection thresholds to the single electron-hole pair level [3–5]
In this paper we have argued that multiple excesses in low-threshold dark matter experiments may be explained by an inelastic excitation, which can be consistently interpreted as a plasmon
To our knowledge this branching fraction has not been calculated in the literature; if our interpretation is correct, such a computation would be highly relevant to DM experiments
Summary
Searches for particle dark matter (DM) with masses below 1 GeV have proliferated in the last decade, driven by advances in detector technologies which have pushed heat detection thresholds below 100 eV [1,2] and charge detection thresholds to the single electron-hole pair level [3–5]. We argue that a compelling explanation of the common ∼10 Hz=kg event rate seen in numerous charge detection experiments, in widely varying background environments, is lacking if the excesses are attributed to plasmon excitation sourced by known Standard Model (SM) particles. These rates can be explained by a common DM origin, albeit through interactions that primarily excite collective charge modes in well-ordered crystals. Further details on dark counts in semiconductor detectors, yield curves, and plasmons are provided in the Appendixes
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