Abstract
In the conventional misalignment mechanism, the axion field has a constant initial field value in the early Universe and later begins to oscillate. We present an alternative scenario where the axion field has a nonzero initial velocity, allowing an axion decay constant much below the conventional prediction from axion dark matter. This axion velocity can be generated from explicit breaking of the axion shift symmetry in the early Universe, which may occur as this symmetry is approximate.
Highlights
Introduction.—Why is CP violation so suppressed in the strong interaction [1,2,3] while near maximal in the weak interaction? The Peccei-Quinn (PQ) mechanism [4,5]
This axion velocity can be generated from explicit breaking of the axion shift symmetry in the early Universe, which may occur as this symmetry is approximate
In this Letter we show that an alternative initial condition for the axion field, θ_ ≠ 0, leads to axion dark matter for larger values of ma
Summary
In the conventional misalignment mechanism, the axion field has a constant initial field value in the early Universe and later begins to oscillate. We present an alternative scenario where the axion field has a nonzero initial velocity, allowing an axion decay constant much below the conventional prediction from axion dark matter This axion velocity can be generated from explicit breaking of the axion shift symmetry in the early Universe, which may occur as this symmetry is approximate. Other known production mechanisms in this mass range are (1) parametric resonance from a PQ symmetry breaking field [39,40], (2) anharmonicity effects [41,42,43] when θi approaches π due to fine-tuning or inflationary dynamics [44,45], (3) decays of unstable domain walls [46,47,48,49,50,51,52,53], and (4) production during a kination era [54] Contrary to these mechanisms, kinetic misalignment offers an exciting theoretical connection with the baryon asymmetry of the Universe through so-called axiogenesis [55]. The nonzero axion velocity corresponds to a rotation of P
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