Axions : Theory and Cosmological Role

Abstract: We review recent developments on axion cosmology. Topics include : axion cold dark matter, axions from topological defects, axion isocurvature perturbation and its non-Gaussianity and axino/saxion cosmology in supersymmetric axion model.

• The paper elucidates the axion solution to the strong CP problem via the Peccei-Quinn symmetry and explores models like KSVZ and DFSZ.
• The paper rigorously analyzes axion production from thermal processes and topological defects, setting key numerical bounds on dark matter density.
• The paper extends its analysis to supersymmetric axion models, highlighting the roles of saxions and axinos in relic abundances and CMB constraints.

An Overview of Axion Cosmology: Theoretical Insights and Cosmological Implications

The paper "Axions: Theory and Cosmological Role" by Masahiro Kawasaki and Kazunori Nakayama explores the intricate theoretical underpinnings and cosmological role of axions, which are hypothesized as solutions to the strong CP problem in quantum chromodynamics (QCD). The paper comprehensively reviews several facets of axion physics, including axion cold dark matter, axions from topological defects, axion isocurvature perturbations, and supersymmetric axion models. This essay aims to outline the significant contributions and theoretical implications presented in the article, potentially guiding future research in this domain.

Axion Solution to the Strong CP Problem

The strong CP problem arises from the QCD Lagrangian's CP-violating term, which inherits constraints due to null experimental results on the neutron electric dipole moment. The Peccei-Quinn (PQ) solution introduces a global U(1) PQ symmetry, spontaneously broken, leading to the emergence of a pseudo-Nambu Goldstone boson—the axion. The axion possesses a potential that dynamically adjusts the CP-violating parameter to zero, effectively addressing the strong CP problem. Various models, such as the KSVZ and DFSZ models, offer frameworks for incorporating the axion into the standard model.

Cosmological Role of Axions

The cosmological implications are significant, as axions provide a compelling dark matter candidate. The paper estimates the axion's relic abundance via multiple production mechanisms, notable among which is the thermal production in the early Universe plasma. Furthermore, axionic strings and domain walls, arising from topological defects during symmetry breaking, also contribute to axion production, with accompanying constraints on the decay constant $F_a$.

Numerical Results and Constraints

The authors provide rigorous numerical analyses of axion emission spectra from axionic strings and domain walls, settling longstanding debates about the character of these distributions. Their findings reveal that the mean axion energy from domain walls is typically a few times the axion mass, adding substantial weight to the cold dark matter density. These contributions are paramount in setting upper bounds on the PQ scale: $F_a \leq (2.0-3.8)\times 10^{10} \,\text{GeV}$ for cases with domain wall number $N_{\rm DW}=1$.

Controversy of Domain Walls and Inflationary Considerations

The paper also addresses the domain wall problem associated with $N_{\rm DW} \ge 2$, which could potentially dominate the Universe. It suggests that explicit breaking of the PQ symmetry or phase alignment during inflation could mitigate these issues. If the PQ symmetry breaks before or during inflation, axions might introduce isocurvature perturbations. The paper evaluates these perturbations against increasingly stringent CMB observations, revealing compatibility for specific inflationary scenarios.

Supersymmetric Axion Models

Extending into supersymmetry, the paper outlines the stabilization of PQ scalars and evaluates cosmological effects of saxions and axinos—the supersymmetric partners of axions. Notably, axino cosmology is intricate due to their interaction and decay channels, influencing relic densities. The paper details potential decay processes and resultant constraints under varying SUSY-breaking scenarios.

Implications and Future Directions

The theoretical and numerical analyses provided point to critical implications for axion models as viable dark matter candidates, reinforcing their relevance against experimental and observational constraints. Notably, the authors' exploration of non-Gaussianities offers a nuanced understanding of CMB constraints. The breadth of considerations for supersymmetric extensions opens paths for future endeavors in model building and phenomenological testing. Proposals for new experiments and simulations are given to probe these theoretical facets, with specific attention to verifying the existence of axions and their role in the Universe's evolution.

In summary, Kawasaki and Nakayama's work composes a detailed and systematic examination of axion cosmology, underscoring the necessity for continued theoretical and observational pursuits to uncover the nature of these enigmatic particles and their imprint on the cosmos.