New scenario of QCD axion clump formation I: Linear analysis

TL;DR

This paper explores new linear scenarios for QCD axion clump formation driven by direct interactions during the QCD phase transition, highlighting potential pathways for axion minicluster development even with early PQ symmetry breaking.

Contribution

It introduces two novel scenarios for axion fluctuation growth during the QCD epoch, expanding understanding of axion minicluster formation mechanisms.

Findings

Rapid axion fluctuation growth possible under enhanced primordial curvature perturbations.

Tuned initial misalignment near the potential hilltop can lead to axion clump formation.

Potential for minicluster formation even if PQ symmetry broke before inflation.

Abstract

The QCD axion acquires the potential through the non-perturbative effect of the QCD matters around the QCD phase transition. During this period, the direct interaction between the axion and the QCD matters sets in. Focusing on the impact of this direct interaction, we propose two scenarios where the fluctuation of the axion can rapidly grow, potentially leading to the formation of axion miniclusters even if the Peccei-Quinn (PQ) symmetry was already broken during inflation. The first scenario assumes that the primordial curvature perturbation at the horizon scale during the QCD epoch was significantly enhanced and the second one assumes that the initial misalignment was tuned around the hilltop of the potential.