Opening up a Window on the Postinflationary QCD Axion

Area of Science:

• Cosmology
• Particle Physics
• Quantum Chromodynamics (QCD)

Background:

• QCD axion cosmology is sensitive to its presence during inflation.
• Standard criteria limit the axion decay constant (f_{a}) relative to the inflationary Hubble scale (H_{I}).

Purpose of the Study:

• To investigate if the Peccei-Quinn (PQ) symmetry can remain unbroken during inflation even when f_{a} >> H_{I}.
• To explore new parameter space for QCD axion dark matter.

Main Methods:

• Analyzing the effect of nonrenormalizable interactions between the PQ scalar field and the inflaton.
• Considering high-dimensional operators that respect inflaton shift symmetry.
• Investigating nonderivative couplings to control inflaton shift symmetry breaking.
• Introducing an early matter domination era.

Main Results:

• The PQ symmetry can remain unbroken during inflation for f_{a} >> H_{I} via inflaton-PQ scalar field interactions.
• This mechanism enlarges the parameter space for QCD axion dark matter.
• The model avoids constraints from axion isocurvature perturbations.

Conclusions:

• A new mechanism allows for a post-inflationary QCD axion with a high decay constant.
• This significantly expands viable parameter space for QCD axion dark matter models.
• Early matter domination can further accommodate high f_{a} values for observed dark matter abundance.