Light QCD axion dark matter from double level crossings

Abstract

The even light QCD axion called the ZN axion can both solve the strong CP problem and account for the dark matter (DM). We point out that the single and double level crossings can naturally take place in the mass mixing between the ZN axion and axionlike particle (ALP). The first level crossing occurs much earlier than the QCD phase transition, while the second level crossing occurs exactly during the QCD phase transition if it exists. We also find that the single level crossing can transform into the double level crossings, depending on the ALP mass mA versus the zero-temperature ZN axion mass ma,0. Compared with the no level crossing case, the ZN axion relic density can be suppressed in the single level crossing, and enhanced or suppressed in the double level crossings.