Natural SUSY with mixed axion/axino dark matter

TL;DR

This paper explores the viability of mixed axion-axino dark matter in natural supersymmetry models, mapping out parameter space where such scenarios align with observed dark matter abundance.

Contribution

It provides a detailed analysis of axino as LSP in natural SUSY, identifying viable regions in PQ scale and axino mass for mixed dark matter.

Findings

Axino mass around 100 keV can produce the correct dark matter abundance.

Solutions with mainly warm axino dark matter are found at f_a~10^{11} GeV.

Higher f_a values yield mainly axion cold dark matter with minimal axino contribution.

Abstract

While supersymmetric models provide a solution to the big hierarchy problem, natural SUSY is also allowed by the little hierarchy problem. In supersymmetric models which include the Peccei-Quinn (PQ) solution to the strong CP problem, one expects the presence of an axion-axino-saxion supermultiplet with a micro-eV-scale axion and a saxion with mass of order the soft breaking scale. The axino mass is much more model-dependent, and may occur in the range of keV-TeV: over 9 orders of magnitude. This leads to the possibility of the axino as lightest SUSY particle (LSP) and the presence of mixed axion plus axino dark matter. The case of natural SUSY with higgsino-like WIMPs as LSP seems (nearly) excluded by multi-ton noble liquid WIMP detector limits, even in the case where the LSP has a depleted abundance compared to axions. We examine the case where the axino is LSP leading to mixed axion-axino dark matter in a natural SUSY context. We map out regions of PQ scale f_a vs. axino mass m_{\ta} parameter space where such a scenario remains viable in both the SUSY DFSZ and KSVZ axion models. For axino mass ~100 keV, we find solutions in accord with the measured dark matter abundance with mainly warm axino dark matter for f_a~ 10^{11} GeV and also solutions with mainly axion cold DM and a tiny axino contribution for higher f_a~ 3\times 10^{12} GeV.