The QCD Axion: A Unique Player in the Axiverse with Mixings

Abstract

In an axiverse with numerous axions, the cosmological moduli problem poses a significant challenge because the abundance of axions can easily exceed that of dark matter. The well-established stochastic axion scenario offers a simple solution, relying on relatively low-scale inflation. However, axions are typically subject to mixing due to mass and kinetic terms, which can influence the solution using stochastic dynamics. Focusing on the fact that the QCD axion has a temperature-dependent mass, unlike other axions, we investigate the dynamics of the QCD axion and another axion with mixing. We find that the QCD axion abundance is significantly enhanced and becomes larger than that of the other axion for a certain range of parameters. This enhancement widens the parameter regions accounting for dark matter. In addition, we also find a parameter region in which both axions have enhanced abundances of the same order, which result in multi-component dark matter.

31pages, 11 figures

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Cited by in corpus (3)

• Induced Domain Walls of QCD Axion, and Gravitational Waves
• Isotropic cosmic birefringence from string axion domain walls without cosmic strings, and DESI results
• Resolving the QCD Axion Domain Wall Problem with a Light Axion