New Perspectives on Axion Misalignment Mechanism

TL;DR

This paper explores a novel axion misalignment mechanism with nonzero initial velocity, showing it can significantly change relic abundance predictions and open new parameter spaces for axion dark matter.

Contribution

It introduces a new scenario where initial axion velocity affects relic abundance, expanding the understanding of axion dark matter models.

Findings

Nonzero initial velocity alters axion relic abundance predictions.

The scenario depends on the timing of PQ symmetry breaking and inflation.

New viable parameter regions for axion dark matter are identified.

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.