Abstract
Dark matter is poorly constrained by direct detection experiments at masses below 1 MeV. This is an important target for the next generation of experiments, and several methods have been proposed to probe this mass range. One class of such experiments will search for dark matter--electron recoils. However, simplified models with new light degrees of freedom coupled to electrons face significant pressure from cosmology, and the extent of these restrictions more generally is poorly understood. Here, we perform a systematic study of cosmological constraints on models with a heavy mediator in the context of an effective field theory. We include constraints from (i) disruption of primordial nucleosynthesis, (ii) overproduction of dark matter, and (iii) the effective number of neutrino species at recombination. We demonstrate the implications of our results for proposed electron recoil experiments, and highlight scenarios which may be amenable to direct detection.
Highlights
The identity of dark matter (DM) remains one of the most significant problems in cosmology and particle physics
(i) The DM must not significantly alter successful predictions of the ratios of light elemental abundances produced in big bang nucleosynthesis (BBN) [32,33,34]; (ii) To accord with measurements of the effective number of neutrino species (Neff), the thermal history of the DM species must not significantly alter the temperature ratio of photons and neutrinos at recombination [35]; (iii) While a single species of DM particle may not account for the entirety of the present-day DM density, no species may be produced with an abundance exceeding that threshold
We revisit the generality of our constraints, point out possible exceptions, and discuss the outlook for sub-MeV DM at electron recoil experiments
Summary
The identity of dark matter (DM) remains one of the most significant problems in cosmology and particle physics. Several well-motivated scenarios (e.g., asymmetric DM, [11]) naturally feature masses between 1 keV and 10 GeV, making this range an appealing target for future direct detection experiments [12] This has driven much interest in novel detection methods suited to light DM particles, and several such experiments have been proposed in the last few years [13,14,15,16,17,18,19,20,21,22,23] (iii) While a single species of DM particle may not account for the entirety of the present-day DM density, no species may be produced with an abundance exceeding that threshold In each case, such cosmological constraints bound the couplings between new species and Standard Model (SM) particles, which determine the event rates in direct detection experiments. When speaking about the DM species generally, without specifying its spin, we will denote it with χ
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