Axions and the Strong CP Problem

TL;DR

<3-5 sentence high-level summary> The paper surveys the strong CP problem and the axion as its leading solution, detailing the theoretical underpinnings of the PQ mechanism, the mass and couplings of the QCD axion, and how various realizations (KSVZ, DFSZ, and string-inspired models) differ in phenomenology. It connects axion physics to cosmology and astrophysics, discussing the allowed Fa window, the role of axions as dark matter, and the implications of anthropic and quintessential scenarios. It also reviews current and proposed detection strategies, including solar helioscopes, haloscopes, and laser-based tests, and extends to very light axions, axions from extra dimensions, and axino cosmology within SUSY frameworks. The work synthesizes particle theory, cosmology, and experimental efforts to map the viability and discoverability of axions across a wide mass and coupling landscape.

Abstract

Current upper bounds of the neutron electric dipole moment constrain the physically observable quantum chromodynamic (QCD) vacuum angle $|\barθ| \lesssim 10^{-11}$. Since QCD explains vast experimental data from the 100 MeV scale to the TeV scale, it is better to explain this smallness of $|\barθ|$ 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 $θ_1$, has been obtained by astrophysical and cosmological data. For $F_a\gtrsim 10^{12}$ GeV with $θ_1<{\cal O}(1)$, 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.

Figures (29)

• Figure 1: The determinental interaction of light quarks. Chiral symmetry breaking introduces the anomalous $\eta'$ mass term from the quark condensations.
• Figure 2: The Lee-Weinberg type plots for (a) the neutrino $\Omega_\nu h^2$KolbTur90 and (b) the axion $\Omega_a h^2$, where $h$ is the present Huble constant in units of $\rm 100\, km\, s^{-1}Mpc^{-1}$. The dash line of (a) is for $\Omega_\nu h^2=0.113$. In (b), it corresponds to the hadronic axion. The blue and red dashlines correspond to the CDM and hot DM limits, respectively.
• Figure 3: Some proposed particles in the interaction cross section versus the corresponding particle mass $m_i$ plane. The skeleton is taken from Ref. Roszfig04. The dashed curves represent schematic shapes of $\Omega_i$ versus the corresponding particle mass $m_i$. The small red square-box corresponds to the hot DM hadronic axion. Two small outside squares (in the cyan and blue colors) in the axion region are marked to show the plausible (GUT and CDM) axions, respectively. The abundances of heavy axino, gravitino and wimpzilla depend on how inflation ends.
• Figure 4: The insertion of the ${\cal C}$${\cal P}$ violation effect by VEVs of $\pi^0$ and $\eta'$ in (a). They can be transferred to one vertex shown as a bullet in (b). With this bullet, the ${\cal C}$${\cal P}$ violation is present by a mismatch between the ${\cal C}$${\cal P}$ conserving RHS vertex and ${\cal C}$${\cal P}$ violating LHS vertex.
• Figure 5: Diagrams contributing to the NEDM with the bullet representing the ${\cal C}$${\cal P}$ violation effect. Diagram (a) is the physically observable contribution.