Abstract
We examine in greater detail the recent proposal of using superconductors for detecting dark matter as light as the warm dark matter limit of O(keV). Detection of such light dark matter is possible if the entire kinetic energy of the dark matter is extracted in the scattering, and if the experiment is sensitive to O(meV) energy depositions. This is the case for Fermi-degenerate materials in which the Fermi velocity exceeds the dark matter velocity dispersion in the Milky Way of ~10^-3. We focus on a concrete experimental proposal using a superconducting target with a transition edge sensor in order to detect the small energy deposits from the dark matter scatterings. Considering a wide variety of constraints, from dark matter self-interactions to the cosmic microwave background, we show that models consistent with cosmological/astrophysical and terrestrial constraints are observable with such detectors. A wider range of viable models with dark matter mass below an MeV is available if dark matter or mediator properties (such as couplings or masses) differ at BBN epoch or in stellar interiors from those in superconductors. We also show that metal targets pay a strong in-medium suppression for kinetically mixed mediators; this suppression is alleviated with insulating targets.
Highlights
As a result of this focus on DM at the weak scale, the sensitivity of experiments to such DM has dramatically increased
Considering a wide variety of constraints, from dark matter self-interactions to the cosmic microwave background, we show that models consistent with cosmological/astrophysical and terrestrial constraints are observable with such detectors
We have explored in detail a proposal for detecting DM with Fermi-degenerate materials, focused on the case of a superconducting metal target
Summary
When searching for DM with mass heavier than ∼ 10 GeV with elastic scattering, nuclear targets have three main advantages: first, DM elastic scattering has a rate that scales as the reduced mass of the DM-target system, μr, which suppresses the scattering rate on electrons compared to that on nucleons. To access even lighter DM, electron targets are preferred In this case, an energy deposition sensitivity of 1 eV corresponds to probing DM models with mass down to roughly 1 MeV. An alternative route towards detecting DM as light as a keV is to take advantage of the gap, with inelasticity catalyzing the scattering This is evident from eq (2.2): even if the first two terms are below a meV, as long as the kinetic energy of the DM exceeds the gap, the DM energy may be absorbed by exciting an electron above the gap.
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