Axion Mass Prediction from Adaptive Mesh Refinement Cosmological Lattice Simulations

Area of Science:

• Cosmology
• Particle Physics
• Astrophysics

Background:

• The quantum chromodynamics (QCD) axion is a hypothetical particle proposed to solve the strong CP problem.
• Its abundance as cold dark matter (DM) depends on its mass, which is linked to the Peccei-Quinn (PQ) symmetry breaking scale.
• Axion strings and their radiation are key to predicting this mass, informing experimental searches.

Purpose of the Study:

• To precisely compute the spectral index of axion radiation emitted by axion strings.
• To refine the prediction of the QCD axion mass based on new simulation results.
• To investigate the impact of the QCD phase transition on axion production.

Main Methods:

• Utilized large-scale simulations of the axion-string network with adaptive mesh refinement.
• Achieved high precision comparable to a static lattice of 262,144^3 sites.
• Analyzed the spectral index of axion radiation from strings.

Main Results:

• Found a scale-invariant axion radiation spectrum with 1% precision.
• Observed no evolution in the spectral index of radiation over time.
• Predicted an axion mass of approximately 45–65 μeV from string radiation before the QCD phase transition.

Conclusions:

• Preliminary evidence suggests increased axion production during the QCD phase transition's string-domain-wall network collapse.
• This could elevate the predicted axion mass to as high as 300 μeV.
• The refined mass prediction has significant implications for ongoing and future dark matter experiments.