Properties of QCD Axion Dark Matter from Cosmological Lattice Simulations

by Mathieu Kaltschmidt (Zaragoza U)

Wednesday 15 Oct 2025, 12:30 → 14:30 America/Los_Angeles

48/2-224 - Madrone (SLAC)

I will present two examples of how cosmological lattice simulations can be used to study the properties of QCD axion dark matter in the early Universe. First, the phenomenology of three variants of the Axion Misalignment mechanism (Standard, Kinetic, and Large Misalignment) will be discussed. It can be shown that non-linearities and the phenomenon of fragmentation depend not only on the non-zero initial velocity $\dot{\theta}_1$ but also explicitly on the initial phase $\theta_1$ of the axion field, with important implications for the resulting miniclusters in the pre-inflationary scenario. The second part of the talk will focus on cold dark matter QCD axions produced from the decay of global cosmic strings in the early Universe, which provide well-motivated experimental targets since their mass can, in principle, be sharply predicted. We confirm the trend that the spectral index $q$ of the axion emission spectrum grows with string tension, but due to several previously overlooked systematic effects, we cannot yet determine whether it continues to increase or saturates at larger values. Accounting for these uncertainties and extrapolating to realistic string tensions with a simple power-law assumption on the spectrum, we predict the axion dark matter mass in the range $m_a \approx 95$–$450\ \mu$eV. Finally, I will highlight recent progress in developing the Kalb–Ramond effective theory for global axion strings, as well as advances in numerical methods, particularly Adaptive Mesh Refinement and Perfectly Matched Layers, that promise to further sharpen these predictions.

Kaltschmidt_SLAC.pdf