Axions and the Strong CP Problem

TL;DR

This paper reviews the theoretical background, models, and recent experimental and observational constraints on axions, particles proposed to solve the strong CP problem and potentially constitute dark matter, highlighting their significance in particle physics and cosmology.

Contribution

It provides a comprehensive overview of axion theory, supersymmetrization, models, and recent data, along with future prospects for axion detection.

Findings

Axions can solve the strong CP problem within specific decay constant ranges.

Astrophysical and cosmological data constrain axion properties and parameter space.

Axions are viable dark matter candidates with ongoing experimental search efforts.

Abstract

Current upper bounds of the neutron electric dipole moment constrain the physically observable quantum chromodynamic (QCD) vacuum angle $|\bar\theta| \lesssim 10^{-11}$. Since QCD explains vast experimental data from the 100 MeV scale to the TeV scale, it is better to explain this smallness of $|\bar\theta|$ in the QCD framework, which is the strong \Ca\Pa problem. Now, there exist two plausible solutions to this problem, one of which leads to the existence of the very light axion. The axion decay constant window, $10^9\ {\gev}\lesssim F_a\lesssim 10^{12} \gev$ for a ${\cal O}(1)$ initial misalignment angle $\theta_1$, has been obtained by astrophysical and cosmological data. For $F_a\gtrsim 10^{12}$ GeV with $\theta_1<{\cal O}(1)$, axions may constitute a significant fraction of dark matter of the universe. The supersymmetrized axion solution of the strong \Ca\Pa problem introduces its superpartner the axino which might have affected the universe evolution significantly. Here, we review the very light axion (theory, supersymmetrization, and models) with the most recent particle, astrophysical and cosmological data, and present prospects for its discovery.