Malte Buschmann: Dark Matter from Axion Strings
Abstract: Axions are hypothetical particles that may explain the observed dark matter (DM) density and the non-observation of a neutron electric dipole moment. An increasing number of axion laboratory searches are underway worldwide, but these efforts are made difficult by the fact that the axion mass is largely unconstrained. If the axion is generated after inflation there is a unique mass that gives rise to the observed DM abundance; due to nonlinearities and topological defects known as axion strings, computing this mass accurately has been a challenge for four decades. Recent works, making use of large static lattice simulations, have led to largely disparate predictions for the axion mass, spanning the range from 25 microelectronvolts to over 500 microelectronvolts. In this talk, I will show that adaptive mesh refinement (AMR) simulations are better suited for axion cosmology than the previously-used static lattice simulations. Using dedicated AMR simulations we obtain an over three orders of magnitude leap in dynamic range and provide evidence that axion strings radiate their energy with a scale-invariant spectrum, to within ∼5% precision, leading to a mass prediction in the range (40,180) microelectronvolts.
Peera Simakachorn: Charting new particle physics with primordial GWs.
Abstract: Particle physics beyond the Standard Model can temporarily induce an equation of state for the early Universe which is not that of radiation. The resulting non-standard expansion histories leave signatures in primordial stochastic backgrounds of gravitational waves (GW). One of the most exciting scenarios is the kination era — the kinetic energy of a scalar field dominates the Universe. This talk focuses on a matter-kination era that can occur naturally inside the standard radiation era due to axion dynamics. The matter-kination era imprints a smoking-gun GW peak, whose position depends on kination's energy scale and duration. Remarkably, the viable parameter space allows for a kination era at the MeV-EeV scale and generates a GW peak from either primordial inflation or local/global cosmic strings, which lies inside LISA, ET, and CE sensitivity windows. Finally, we connect the matter-kination peak to the abundance of axion dark matter.