[Paper Review] Revisiting the Dark Matter Interpretation of Excess Rates in Semiconductors

Peter Abbamonte, D. Baxter+5 more|arXiv (Cornell University)|Feb 7, 2022

Dark Matter and Cosmic Phenomena39 references7 citations

TL;DR

This paper re-evaluates the possibility that the excess event rates observed in low-threshold semiconductor calorimeters—SuperCDMS CPD (silicon) and EDELWEISS-Surf (germanium)—arise from a common dark matter origin. Despite a consistent power-law spectral index (α = 3.43+0.11−0.06) and normalization scaling with A², the authors rule out a common dark matter scattering origin due to unphysically high required dark matter velocities to explain both excesses, and also exclude known particle sources like cosmic-ray neutrons, photons, and neutrinos based on flux constraints.

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.

Motivation & Objective

• To investigate whether the excess event rates in SuperCDMS CPD (silicon) and EDELWEISS-Surf (germanium) detectors share a common origin.
• To assess the viability of inelastic dark matter scattering via nuclear recoils as an explanation for both excesses.
• To test whether known particle sources—cosmic-ray neutrons, photons, and solar neutrinos—could account for the observed rates.
• To evaluate the role of exotic condensed matter effects in enabling a common dark matter interpretation.
• To resolve the persistent mystery of calorimetric excesses in semiconductor detectors by quantitatively constraining competing models.

Proposed method

• Performs a joint fit to the energy spectra of the SuperCDMS CPD and EDELWEISS-Surf excesses, assuming a power-law form with index α.
• Uses a phenomenological model of detector response parametrized by the dynamic structure factor to describe inelastic nuclear scattering.
• Calculates required dark matter velocities to reproduce the observed rates in both detectors, comparing them to kinematically allowed ranges.
• Evaluates known fluxes of cosmic-ray neutrons, photons, and solar neutrinos to assess their potential contribution to the excesses.
• Applies constraints from XENON1T and other experiments to rule out fast dark matter subcomponents.
• Considers the impact of exotic condensed matter effects, such as those from the Migdal effect, in the unmeasured kinematic regime.

Experimental results

Research questions

• RQ1 Can a common dark matter scattering mechanism explain the excess rates in both silicon and germanium calorimetric detectors?
• RQ2 Is the observed spectral consistency (α = 3.43+0.11−0.06) compatible with a single inelastic dark matter scattering process?
• RQ3 What dark matter velocities are required to produce the observed rates in both detectors, and are they physically viable?
• RQ4 Can known particle sources—cosmic-ray neutrons, photons, and solar neutrinos—account for the observed excesses?
• RQ5 Are exotic condensed matter effects sufficient to reconcile the required kinematics for a common dark matter origin?

Key findings

• The observed energy spectra in SuperCDMS CPD and EDELWEISS-Surf exhibit a consistent power-law index of α = 3.43+0.11−0.06, suggesting a possible common origin.
• The normalization of the excess rates scales approximately with A², consistent with a common scattering process.
• A common dark matter scattering origin is ruled out due to unphysically high required dark matter velocities—exceeding 1000 km/s in the lab frame—necessary to produce the germanium rate at higher energies.
• The possibility of inelastic scattering via exotic condensed matter effects is disfavored due to the extreme kinematic requirements.
• Known particle sources, including cosmic-ray neutrons, photons, and solar neutrinos, are quantitatively ruled out as explanations due to their measured fluxes being too low.
• The calorimetric excesses remain unexplained, with no viable interpretation found among the tested models.