Production of dark matter axions from collapse of string-wall systems (1202.5851v3)

Abstract: We analyze the spectrum of axions radiated from collapse of domain walls, which have received less attention in the literature. The evolution of topological defects related to the axion models is investigated by performing field-theoretic lattice simulations. We simulate the whole process of evolution of the defects, including the formation of global strings, the formation of domain walls and the annihilation of the defects due to the tension of walls. The spectrum of radiated axions has a peak at the low frequency, which implies that axions produced by the collapse of domain walls are not highly relativistic. We revisit the relic abundance of cold dark matter axions and find that the contribution from the decay of defects can be comparable with the contribution from strings. This result leads to a more severe upper bound on the axion decay constant.

• The paper demonstrates via lattice simulations that collapsing string-wall systems emit a low-frequency spectrum of axions that significantly contribute to dark matter.
• It constrains the axion decay constant to less than approximately 2.0×10^10 GeV, challenging traditional limits in axion models.
• The findings underscore the need to account for both cosmic strings and domain walls to accurately assess axion production in the early universe.

Production of Dark Matter Axions from Collapse of String-Wall Systems

The research presented by Hiramatsu et al. investigates the production of dark matter axions originating from the collapse of domain wall systems attached to cosmic strings. This problem exists within the context of the Peccei-Quinn (PQ) mechanism, which offers a prominent solution to the strong CP problem in quantum chromodynamics (QCD). The axions produced as a result of this mechanism are viable candidates for cold dark matter. By utilizing a combination of theoretical insights and numerical simulations, the authors provide a detailed analysis of axion production processes and address discrepancies from previous studies.

Background and Context

The Peccei-Quinn mechanism proposes axions as solutions to the CP violation issue in QCD via spontaneous symmetry breaking of a global U(1) symmetry at a high-energy scale with the axion decay constant $F_a$. This leads to the formation of pseudo-Nambu-Goldstone bosons, the axions. The PQ symmetry breaking induces topological defects, notably cosmic strings and domain walls, in the early universe. The dynamics and decay of these defects are essential to understanding axion production and their contribution to the dark matter density.

Methodology

The researchers employed field-theoretic lattice simulations to quantitatively paper topological defect networks and their evolution, focusing on string-wall systems where strings are bounded by domain walls. They simulated the entire process from the formation of global strings and domain walls to their annihilation initiated by wall tension. A key aspect of the paper is the determination of the axion spectrum emitted from these decaying networks, examining if the generated axions are relativistic and their subsequent impact on axion density in the universe.

Results

Through rigorous simulations, the research establishes that:

1. The spectrum of axions from domain walls peaks at lower frequencies, indicating that these axions are not highly relativistic.
2. The relic density of axions from domain walls can be comparable to that from strings, challenging previous constraints on the axion decay constant.
3. The abundance analyses, supported by numerical results, constrain the axion decay constant $F_a$ to be less than approximately $2.0 \times 10^{10}$ GeV, refining the classical "axion window".
4. This tighter constraint suggests a significant role for axions in dark matter composition, with potential implications for experimental axion searches.

Implications and Future Directions

The paper's findings underscore the importance of considering both string and domain wall contributions in dark matter axion models. The deviated constraints on $F_a$ suggest necessary adjustments in both observational strategies for axions and theoretical models of symmetry breaking. Future research could focus on enhancing simulation precision, extending dynamical ranges, and incorporating larger classes of axion models to capture variations in axion production rates. Furthermore, integrating these simulations with observational data could refine understanding of cosmic string-wall interactions and their cosmological effects.

Overall, this investigation addresses critical gaps in the quantification of axion production, advocating for a comprehensive approach in theoretical and experimental axion physics to resolve outstanding issues in cosmic axion abundances.