The Axion Helical Misalignment Mechanism

Abstract

Understanding axion production in the early Universe remains a pivotal challenge, given the axion as a compelling cold dark matter candidate. Conventional misalignment scenarios often overlook the possibility that a large initial axion velocity can fundamentally reshape the subsequent evolution of the axion field. In this letter, we provide a comprehensive analysis of how primordial magnetic fields impact the axion relic abundance. By accounting for the axion coupling to the Chern-Simons term of the hypercharge gauge field, the equation of motion of the axion is recast as a driven oscillator equation. This modification effectively shifts the onset of axion oscillations, leading to a significant reevaluation of the final relic abundance, a novel effect we term the axion helical misalignment mechanism. Furthermore, in the presence of primordial chiral asymmetries, the chiral magnetic effect (CME) emerges as a critical driver of axion dynamics. The interplay between the axion field and the CME not only profoundly influences the evolution of Standard Model chiral fermions but also provides a viable pathway for generating the observed baryon asymmetry of the Universe.

Figures (2)

• Figure 1: Left-panel: Evolution of total physical helicity $h_{\rm phy}$ and the physical right-handed electron chemical potential $\mu_e^{\rm phy}$. The dashed line indicates negative values; Right-panel: Evolution of total physical energy density $\rho_{B}^{\rm phy}$ as the function of the temperature.
• Figure 2: Left-panel: Evolution of $\partial_t\theta$ as the function the temperature; Right-panel: Relic abundance of the axion as the function of the initial physical helicity $h_{\rm rh}^{\rm phy}$.