Axions as Hot and Cold Dark Matter

Understand

The presence of a hot dark matter component has been hinted at 3 sigma by a combination of the results from different cosmological observations.

• We examine a possibility that pseudo Nambu-Goldstone bosons account for both hot and cold dark matter components.
• We show that the QCD axions can do the job for the axion decay constant f_a < O(10^10) GeV, if they are produced by the saxion decay and the domain wall annihilation.
• We also investigate the cases of thermal QCD axions, pseudo Nambu-Goldstone bosons coupled to the standard model sector through the Higgs portal, and axions produced by modulus decay.