Pi in the Sky: Axion Condensation in Neutron Stars

The QCD axion is one of the most elegant solutions to the strong CP problem and for some masses could also be a constituent of dark matter. Although axion couplings to photons are well-studied in the laboratory, the defining coupling of the QCD axion to gluons is less constrained. In the dense matter of a neutron star, the effective mass of an axion that is coupled to gluons may become negative, leading to the axion field condensing with the QCD-theta angle taking on a value of pi in the core of the neutron star. This can have striking effects on neutron star structure, leading to neutron stars with different phases than ordinarily expected. I will review the effects that axion condensation has on the properties of nucleons and hadrons and the novel features of nuclear forces at reduced pion mass. For "exceptionally light" QCD axions, neutron star observations can exclude much of the parameter space. I will then present ongoing research aiming to constrain axions with mass given by the QCD prediction.