The QCD Axion in Hot and Dense Matter

Within the framework of the two-flavor Nambu-Jona-Lasinio model, we study the quantum chromodynamics axion properties at finite temperature and chemical potential.Our analysis shows that the axion properties are strongly influenced by the critical behavior of the chiral phase transition.In particular, the axion mass follows the response of the chiral condensate to temperature and chemical potential, decreasing monotonically as either parameter increases.Moreover, we observe that at relatively low temperatures, the axion self-coupling constant exhibits a sharp peak near the critical point when the chemical potential increases, reaching more than twice its vacuum value.Beyond the critical point, the self-coupling rapidly decreases to values much smaller than in vacuum, eventually approaching zero at high chemical potential.These results suggest that the significant enhancement of the axion self-coupling in dense matter near the chiral phase transition may favor the formation or amplification of an axion Bose-Einstein condensate in compact astrophysical objects.