Improved Hot Dark Matter Bound on the QCD Axion

Area of Science:

• Cosmology
• Particle Physics

Background:

• Axions are hypothetical particles proposed as dark matter candidates.
• Accurate cosmological bounds on axion mass are crucial for understanding the early universe and particle physics beyond the Standard Model.

Purpose of the Study:

• To derive a reliable cosmological bound on the axion mass (m_a).
• To constrain the sum of neutrino masses (∑m_ν) in the presence of relic axions and neutrinos.
• To identify requirements for future axion detection experiments.

Main Methods:

• Derived axion production rates directly from pion-pion scattering data, bypassing chiral perturbation theory limitations.
• Incorporated momentum dependence into Boltzmann equations for axion-pion scatterings to refine relic abundance calculations.
• Utilized current cosmological datasets to establish mass bounds.

Main Results:

• Obtained a cosmological bound of m_a ≤ 0.24 eV at 95% confidence level.
• Constrained the sum of neutrino masses to ∑m_ν ≤ 0.14 eV at 95% confidence level.
• Identified a ~30% difference from previous axion mass estimates and a ~40% enhancement in relic abundance due to refined methods.

Conclusions:

• The study provides robust cosmological constraints on axion and neutrino masses.
• Reliable nonperturbative calculations above the QCD crossover are essential for future cosmological surveys aiming for axion detection.