Supersymmetry, Axions and Cosmology

TL;DR

The paper argues that in supersymmetric theories, saxion cosmology can drive the viability of axions by relaxing the conventional upper bound on $f_a$ when moduli are heavy and decay before nucleosynthesis, potentially allowing $f_a$ up to about $10^{15}$ GeV and making axions viable dark matter candidates. It surveys axion multiplet models and broader moduli cosmology, highlighting that resolving the cosmological moduli problem often resolves or weakens the axion problem, though the pseudoscalar moduli can still pose severe cosmological challenges in general. The authors discuss mechanisms for stabilizing moduli (e.g., Kahler stabilization, racetrack) and show that the axion-like pseudoscalar can be significantly lighter than its scalar partner, with masses suppressed by nonperturbative dynamics and potentially requiring discrete symmetries to ensure a QCD axion. The work emphasizes that axion phenomenology in SUSY is highly model-dependent and that the usual $f_a$ bounds may not apply universally, especially in gauge-mediation contexts.

Abstract

Various authors have noted that in particular models, the upper bound on the axion decay constant may not hold. We point out that within supersymmetry, this is a generic issue. For large decay constants, the cosmological problems associated with the axion's scalar partner are far more severe than those of the axion. We survey a variety of models, both for the axion multiplet and for cosmology, and find that in many cases where the cosmological problems of the saxion are solved, the usual upper bound on the axion is significantly relaxed. We discuss, more generally, the cosmological issues raised by the pseudoscalar members of moduli multiplets, and find that they are potentially quite severe.