HEP In the literature: contested

What microscopic process produces the excess in cryogenic calorimeters?

In plain words

Crystals cooled near absolute zero record many more small heat pulses below a few hundred electron volts than expected, and the rate fades over weeks after cooling. The question is what inside the detector releases this energy.

Precise statement

Cryogenic calorimeters (CRESST CaWO4, Al2O3, Si; SuperCDMS-type Si and Ge; NUCLEUS; EDELWEISS) observe a rate $\mathrm{d}R/\mathrm{d}E$ rising steeply below about 200 eV, uncorrelated with particle backgrounds and decaying after cooldown. Candidate sources include stress relaxation at holders, glue and sensor films; dislocation motion from differential thermal contraction; and annealing of radiation-induced lattice defects. An answer is a mechanism that reproduces the energy spectrum, the time dependence, the material dependence and the sensor-geometry dependence, confirmed by a controlled test that turns the excess on or off.

What would settle it

A detector in which removing or adding the hypothesized source (e.g., stress in the holder or film, or a defect population) changes the excess rate by a predicted factor.

Status in the literature

Unverified note

In 2026 NUCLEUS (Eur. Phys. J. C 86, 831) found slower cooldown lowers the initial rate and the rate decays as a power law independent of particle background; stress-relaxation (Nature Commun. 2024), thermal-contraction (2026 preprint) and defect-annealing (Phys. Rev. Mater. 2025) explanations all remain under discussion.