Light Axion Dark Matter: Old Challenges and New Ideas

The strong CP problem—the puzzle of why quantum chromodynamics (QCD) appears to conserve CP symmetry despite the weak interactions breaking it—suggests that new physics beyond the Standard Model is required. Promoting the CP phase to a dynamical field, the axion, not only solves the strong CP problem but also provides distinctive experimental and astrophysical signatures, making this a very promising solution to this puzzle. In the standard scenario, a QCD axion with a mass of a few micro-electronvolts can naturally account for dark matter via the misalignment mechanism, thereby making up roughly 25% of the Universe's energy budget. However, both theoretical and observational considerations tend to favor an axion with a much lighter mass, which would typically overproduce dark matter. To resolve this discrepancy, the talk introduces two innovative theoretical frameworks. In the first model, a dark-sector population of magnetic monopoles interacts with the axion through the Witten effect, dynamically reducing the axion dark matter abundance to levels consistent with observations. In the second model, the cosmological challenges of the light ZN-axion scenario are overcome by introducing a reheaton that couples to the Standard Model Higgs, incidentally modifying the canonical misalignment mechanism. We conclude by outlining current work and future directions to pursue.