Thermal Dark Matter Below a MeV

Area of Science:

• Cosmology
• Particle Physics
• Astrophysics

Background:

• Constraints on dark matter (DM) properties are crucial for understanding the universe.
• The effective number of neutrino species provides a key probe of new relativistic particles during Big Bang nucleosynthesis.

Purpose of the Study:

• To investigate a class of models where thermal dark matter is lighter than a MeV.
• To explore the implications of dark matter thermalizing with the standard model before neutrino-photon decoupling.
• To demonstrate a specific model compatible with cosmological and astrophysical observations.

Main Methods:

• Consideration of thermal dark matter models lighter than 1 MeV.
• Analysis of dark matter-standard model thermalization below the neutrino-photon decoupling temperature.
• Demonstration using a model with fermionic dark matter and a light scalar mediator.

Main Results:

• Thermal dark matter lighter than MeV can equilibrate and freeze-out with the standard model.
• This process leads to comparable cooling and heating of the standard model bath.
• Alleviates constraints from the effective number of neutrino species measurements.
• Thermal dark matter as light as a few keV is compatible with cosmological and astrophysical data.

Conclusions:

• The proposed framework supports the existence of sub-MeV thermal dark matter.
• This research motivates the development of new experiments for direct detection of light dark matter and light force carriers.