The Cosmological Axino Problem

TL;DR

This paper examines the cosmological challenges of the supersymmetric QCD axion, revealing a significant axino overproduction problem similar to the gravitino issue, which constrains reheating temperatures and impacts dark matter models.

Contribution

It identifies the axino problem as analogous to the gravitino problem and analyzes the combined constraints on reheating temperature and axion decay constant in supersymmetric cosmology.

Findings

Reveals axino mass must be ≥ gravitino mass without fine-tuning.

Sets upper bounds on reheating temperature T_R < 10^3 - 10^6 GeV.

Proposes three dark matter scenarios with gravitino LSP and axino NLSP.

Abstract

We revisit the cosmology of the supersymmetric QCD axion, highlighting the existence of a serious cosmological axino problem that is fully analogous to the gravitino problem of overclosure via thermal production. A general analysis implies that the QCD axino has a mass greater than or equal to that of the gravitino in the absence of unnatural fine-tuning or sequestering. As a consequence, bounds from thermal gravitino and QCD axino production are complementary in parameter space, and together provide a quite stringent limit on the reheating temperature after inflation given by T_R < 10^3 - 10^6 GeV for an axion decay constant of f_a = 10^9 - 10^12 GeV. Motivated by this result, we explore the cosmology of gravitino LSP and axino NLSP at low T_R and present three realistic scenarios for dark matter.