Abstract
We consider a class of models in which thermal dark matter is lighter than a MeV. If dark matter thermalizes with the standard model below the temperature of neutrino-photon decoupling, equilibration and freeze-out cool and heat the standard model bath comparably, alleviating constraints from measurements of the effective number of neutrino species. We demonstrate this mechanism in a model consisting of fermionic dark matter coupled to a light scalar mediator. Thermal dark matter can be as light as a few keV, while remaining compatible with existing cosmological and astrophysical observations. This framework motivates new experiments in the direct search for sub-MeV thermal dark matter and light force carriers.
Highlights
The expectation of new physics at the electroweak scale has motivated the search for dark matter (DM) in the form of weakly interacting massive particles (WIMPs) with mass 10 GeV ≲ mDM ≲ 1 TeV
It has been argued that any subMeV relic that is in thermal contact with the standard model (SM) below the temperature of neutrino decoupling necessarily leads to measurable deviations in the expansion rate of the Universe [8,9,10,11,12,13,14,15]
The value of Neff is constrained by the successful predictions of big bang nucleosynthesis (BBN) and observations of the cosmic microwave background (CMB)
Summary
SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA (Received 6 July 2017; revised manuscript received 15 November 2017; published 8 January 2018). If dark matter thermalizes with the standard model below the temperature of neutrino-photon decoupling, equilibration and freeze-out cool and heat the standard model bath comparably, alleviating constraints from measurements of the effective number of neutrino species. Our key observation is that if a light state enters thermal equilibrium with the SM after neutrino-photon decoupling, the constraints from measurements of Neff during BBN and recombination are significantly relaxed. If this light state couples only to neutrinos, equilibration draws heat from the SM bath without changing Neff. The minimal model that we investigate does not give rise to appreciable signals at proposed direct
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