Revisiting the Dark Matter Interpretation of Excess Rates in Semiconductors

TL;DR

This paper reevaluates the potential dark matter explanation for excess signals in semiconductor detectors, finding that common nuclear recoil scenarios are unlikely due to velocity constraints and disfavoring cosmic ray and neutrino origins.

Contribution

It provides a detailed analysis of excess rates in semiconductor detectors, demonstrating that a dark matter origin via nuclear recoils is unlikely and exploring alternative explanations.

Findings

Observed excess spectra follow a power law with index ~3.43.

Dark matter scattering via nuclear recoils is strongly disfavored.

Cosmic ray neutrons, solar neutrinos, and photons are unlikely sources.

Abstract

In light of recent results from low-threshold dark matter detectors, we revisit the possibility of a common dark matter origin for multiple excesses across numerous direct detection experiments, with a focus on the excess rates in semiconductor detectors. We explore the interpretation of the low-threshold calorimetric excess rates above 40 eV in the silicon SuperCDMS Cryogenic Phonon Detector and above 100 eV in the germanium EDELWEISS Surface detector as arising from a common but unknown origin, and demonstrate a compatible fit for the observed energy spectra in both experiments, which follow a power law of index $\alpha = 3.43^{+0.11}_{-0.06}$. Despite the intriguing scaling of the normalization of these two excess rates with approximately the square of the mass number $A^2$, we argue that the possibility of common origin by dark matter scattering via nuclear recoils is strongly disfavored, even allowing for exotic condensed matter effects in an as-yet unmeasured kinematic regime, due to the unphysically-large dark matter velocity required to give comparable rates in the different energy ranges of the silicon and germanium excesses. We also investigate the possibility of inelastic nuclear scattering by cosmic ray neutrons, solar neutrinos, and photons as the origin, and quantitatively disfavor all three based on known fluxes of particles.