Particle Physics Seminar: Systematic approach to axion production at finite density

Abstract:

The QCD axion is one of the best-motivated particles beyond the SM. On the phenomenological side, it is extremely predictive as its mass and its couplings to SM particles are determined by a single scale: the axion decay constant. As of today, astrophysical observations, such as neutron star cooling and energy loss from supernovae, place the strongest bounds.

In this talk, I will present how these astrophysical constraints are modified when we account for changes in the derivative axion-nucleon coupling in dense and high-temperature environments—an effect that has previously been overlooked. Using chiral perturbation theory, I will show how loop corrections induce an energy dependence to the derivative axion-nucleon coupling and, in addition, how this coupling varies with the nucleon Fermi momentum. As a consequence, the supernova bound for the KSVZ axion is strengthened by one order of magnitude. Then, I will show that this modification also significantly strengthens the constraint on the axion neutron coupling from neutron star cooling. 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 that arises 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.

Seminar Organizer: Dr. Michael Geller