ABSTRACT
Axions are hypothetical particles whose existence would explain what is otherwise a puzzling feature of quantum chromodynamics (QCD), the leading theory of strong interactions. QCD contains a dimensionless free parameter whose value must be ‘unnaturally’ small in order for the theory not to violate a combination of charge conservation and mirror-symmetry known as charge parity. Axions, if they exist, meet all the requirements of a successful cold dark-matter or CDM candidate. This chapter calculates the bolometric intensity of decaying axions and finds that axions, if they are to make up the CDM, must be either exceedingly light or exceedingly weakly coupled. Thermal axions in the multi-eV window remain (only just) viable at the lightest end of the range of possible rest-masses. They may also exist with slightly higher rest-masses, but only in certain axion theories where their couplings to photons are weak.
