Does the Migdal effect occur at the predicted rate in detector media?
In plain words
When a nucleus is hit suddenly, its electrons may be shaken loose, giving a detectable signal even from very light dark matter; this is the Migdal effect. It has now been seen directly, but searches in liquid xenon have not found it at the predicted rate.
Precise statement
The Migdal effect is ionization or excitation of atomic electrons following an abrupt nuclear recoil, with probability predicted by Ibe et al. (2018) for isolated atoms and by solid-state calculations for crystals. The quantity is the ratio of the Migdal ionization cross section to the elastic nuclear-recoil cross section, measured with neutron beams, in liquid xenon, liquid argon and silicon or germanium crystals, compared with theory. An answer is this ratio to about 20 percent in each medium.
What would settle it
Neutron-scattering measurements with tagged nuclear recoils in each target medium, extracting the Migdal rate per recoil and its electron-energy spectrum.
Status in the literature
Unverified note
Yi et al. (Nature 649, 580, 2026) reported a $5\sigma$ observation in neutron scattering (6 candidate events) with Migdal to nuclear-recoil cross-section ratio $4.9 (+2.6 -1.9)e-5$; a liquid xenon search (Xu et al., Phys. Rev. D 109, L051101, 2024) saw no signal at the predicted level, and an LZ thesis analysis quotes a 90 percent limit at 26 percent of the Ibe et al. prediction.