Konstantin Springmann (TUM)

4:00pm Wednesday April 21, 2021

Axions in dense objects

In this talk I will show how the properties of axion-like particles (ALPs) coupled to QCD change in systems at finite baryonic density, such as neutron stars. For instance, at nuclear saturation density, the mass of the QCD-axion is mildly reduced and its model-independent coupling to neutrons enhanced by up to an order of magnitude. At moderately higher as well as ultra-high densities, if realized meson (kaon) condensation can trigger axion condensation. In general, the phenomenon of ALP condensation can have significant implications for the composition, stability and emissivity of neutron stars. I will focus on a particularly thrilling possibility that arises for ALPs whose ground state in vacuum is metastable: dense enough objects can destabilise the minimum and lead to the formation of a bubble that expands throughout the entire universe. Such seeded phase transitions allow us to set new constraints on the parameter space of benchmark relaxion models