Axions in extreme environments: from white dwarfs to supernovae

by Konstantin Springmann (TUM)

Thursday Nov 13, 2025, 2:00 PM → 3:00 PM Europe/Paris

Auditorium (LAPTh)

The QCD axion and axion-like particles are compelling dark matter candidates and can solve the strong CP problem. In this talk, I will demonstrate how fundamental properties of QCD axions, such as their potential and couplings to Standard Model particles, depend on finite density and temperature.
Some of the strongest constraints on axions come from neutron star and supernova cooling. By employing chiral perturbation theory at finite density, I will demonstrate how the relevant axion-nucleon coupling is modified within dense environments. As a consequence, the supernova bound for the KSVZ axion is slightly strengthened. For models in which the derivative axion-nucleon coupling is suppressed, "astrophobic" axion models, I will point out a novel axion production channel from a non-derivative operator present for all QCD axion models at next-to-leading order in the chiral expansion. The resulting axion luminosity is four orders larger compared to processes that have been considered previously.
Furthermore, the presence of a finite density background, such as found in stars and planets, reduces the axion mass, which has a series of phenomenological implications. First, if the axion constitutes DM, the axion mass reduction leads to distortions of the axion DM background with important consequences for terrestrial experiments. Secondly, if the mass reduction dominates, the axion can get displaced from its zero-density minimum, which can reduce the masses of nucleons by ~O(10)MeV within the object. I will show that this changes the equation of state of compact objects such as white dwarfs and neutron stars, which leads to some of the strongest constraints in the axion parameter space.