Can detectors reach the sub-MeV freeze-in dark matter benchmark?
In plain words
In one favored scenario, light dark matter was slowly produced in the early universe through a very light force carrier, which fixes how often it should scatter off electrons. Silicon detectors now exclude this scenario over much of the MeV to GeV range, but lighter dark matter carries too little energy to free an electron in silicon.
Precise statement
In the freeze-in benchmark with an ultralight dark-photon mediator, the DM-electron cross section $\sigma_e(m_{\chi})$ is fixed by the relic abundance. For $m_{\chi}$ below about 1 MeV the kinetic energy $(1/2) m_{\chi} v^2$ with $v \sim 1e-3 c$ is below the 1.1 eV silicon gap, so semiconductor targets lose sensitivity. The question is whether a target with meV-scale gap or collective response (superconductors, Dirac materials, polar crystals via in-medium dark-photon mixing, superconducting nanowires) reaches the freeze-in line for $m_{\chi} = 10\,\mathrm{keV} - 1\,\mathrm{MeV}$, accounting for in-medium screening.
What would settle it
An exposure-scaled limit from a low-gap target that crosses the freeze-in line, with in-medium dielectric response measured or computed for that material.
Status in the literature
Unverified note
DAMIC-M (Phys. Rev. Lett. 135, 071002, 2025) excluded freeze-in and freeze-out benchmarks over large ranges below 1 GeV; QROCODILE (Phys. Rev. Lett. 2025) set first sub-MeV limits with a 0.11 eV threshold, still far from the sub-MeV freeze-in line.