What causes the bulk ionization excess in DAMIC CCDs at SNOLAB?
In plain words
Silicon camera chips used to search for dark matter deep underground see extra small charge signals from inside the silicon, beyond all known radioactivity. The cause is unknown.
Precise statement
DAMIC at SNOLAB reported an excess of bulk ionization events above the radiogenic background model, with characteristic rate about 7 events/(kg day) at electron-equivalent energies about 80 eV, later confirmed with skipper CCDs. An answer identifies the source (unmodeled background such as low-energy Compton or surface events, a detector effect, or new physics) and predicts the rate and spectrum in other silicon detectors.
What would settle it
Comparison of the bulk excess across silicon CCDs with different fabrication, depth, shielding and cosmogenic exposure (DAMIC-M, SENSEI, Oscura prototypes) and with calibrated low-energy Compton spectra.
Status in the literature
Unverified note
Confirmed with skipper CCDs at SNOLAB (Phys. Rev. D 109, 062007, 2024, arXiv 2306.01717); origin unknown.