New perspectives on axion misalignment mechanism

Chia-Feng Chang,Yanou Cui

doi:10.1103/physrevd.102.015003

Abstract

A zero initial velocity of the axion field is assumed in the conventional misalignment mechanism. We propose an alternative scenario where the initial velocity is nonzero, which may arise from an explicit breaking of the PQ symmetry in the early Universe. We demonstrate that, depending on the specifics about the initial velocity and the time order of the PQ symmetry breaking vs. inflation, this new scenario can alter the conventional prediction for the axion relic abundance in different, potentially significant ways. As a result, new viable parameter regions for axion dark matter may open up.

Highlights

Axions are ultralight pseudoscalar particles that are generically predicted in the Peccei-Quinn (PQ) mechanism [1,2,3], a compelling solution for the strong CP problem in particle physics
For postinflationary PQ symmetry breaking, axion topological defects necessarily form and through their subsequent decays may contribute to axion relic abundance (Ωa) in significant ways [10,11,12,13,14,15]
We propose a new misalignment mechanism where the axion initial velocity is nonzero and demonstrate its impact on axion relic abundance based on systematically classified benchmark cases with a UV model-independent approach

Summary

INTRODUCTION

Axions are ultralight pseudoscalar particles that are generically predicted in the Peccei-Quinn (PQ) mechanism [1,2,3], a compelling solution for the strong CP problem in particle physics. Our understanding of possible outcomes of the misalignment production may not be complete either. How would a nonzero initial velocity of the axion field influence the axion relic abundance and phenomenology?. Based on classified benchmark examples in a UV model-independent approach [24], we demonstrate the conditions when axion relic density prediction can significantly differ from the conventional, with potentially dramatic enhancement or suppression depending on specifics with θ_i and whether the PQ symmetry breaks before, during, or after inflation. An example model realizing such an initial condition is illustrated in Appendix B Another recent work [25] considered the possibility of θ_i ≠ 0, focusing on the large θ_ > 0 region, demonstrating examples of interesting UV complete models leading to an enhanced Ωa

THE ORIGIN OF A NONZERO INITIAL VELOCITY

AXION MISALIGNMENT MECHANISM WITH A NONZERO INITIAL VELOCITY

CONCLUSION