Can QCD Axions Survive the Cosmological Constant Problem?

TL;DR

This paper investigates how vacuum energy relaxation mechanisms impact QCD axions, showing that such models can significantly alter axion properties and potentially rule out standard QCD axions as viable dark matter candidates.

Contribution

It demonstrates that vacuum energy relaxation models can modify axion cosmology, challenging the viability of standard QCD axions within these frameworks.

Findings

Relaxation suppresses the axion vacuum potential, changing its cosmological behavior.

The axion mass-coupling relation is shifted away from the standard QCD band.

Standard QCD axions may be incompatible with vacuum-energy relaxation models.

Abstract

Mechanisms that dynamically relax the vacuum energy offer a concrete way to approach the cosmological constant problem, but because relaxation is not confined to the vacuum energy alone it can have consequences for the rest of low-energy physics. We explore this issue using the recently proposed 'yoga' relaxation models as an explicit framework and show how relaxation differentially suppresses 'slow' physics relative to a characteristic timescale set by the mass of the relaxon. It therefore need not alter e.g. Higgs & collider physics but can dramatically change how light scalar fields participate in cosmology. We revisit the QCD axion in this setting and show that the suppression of the axion's vacuum potential reshapes its behaviour on cosmological timescales while leaving fast, high-energy processes unaffected. The result is to alter the axion mass-coupling relation away from the standard QCD band, driving it into a regime already ruled out by observational constraints. In particular, suppression of the vacuum axion potential allows the QCD matter-induced potential to dominate even for matter densities relevant to cosmology and everyday matter, potentially driving the axion away from the CP-conserving minimum for QCD-motivated parameters. We conclude that conventional QCD axions are unlikely to remain viable in their standard form within vacuum-energy relaxation frameworks.