Axions and the Strong CP Problem (0807.3125v2)

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.

• The paper presents a theoretical framework using Peccei-Quinn symmetry to resolve the strong CP problem dynamically.
• Axion models are analyzed with astrophysical limits, predicting masses and decay constants that refine dark matter parameters.
• Laboratory methods like helioscopes and cavity experiments are discussed as indirect detection strategies guiding future research.

Overview of "Axions and the Strong CP Problem"

The academic paper titled "Axions and the Strong CP Problem" by Jihn E. Kim and Gianpaolo Carosi explores the theoretical framework related to the axion—a hypothetical elementary particle—and its connection to solving the strong CP problem in quantum chromodynamics (QCD). This problem arises from the nonzero vacuum angle θ in QCD, which leads to a CP violation that isn't observed in physical experiments. The existence of such a small angle implies a need for theoretical intervention to mitigate the inconsistency in the observed phenomena.

Axions and QCD

The strong CP problem is predominantly addressed by introducing the Peccei-Quinn (PQ) symmetry, which posits the axion as a solution. Axions are proposed pseudoscalar particles that arise from the breaking of this symmetry and are incorporated to account for the mysterious smallness of θ. Theoretical models predict that axions should have an extremely small mass given that their existence tries to restore observable CP conservation by dynamically setting θ to zero.

Model Predictions and Astrophysical Implications

The paper goes into depth on the prediction of axion masses and decay constants, highlighting constraints that arise from cosmological and astrophysical observations. Axions, if they exist in the predicted mass range, could be a significant component of cold dark matter (CDM) in the universe. Axion models suggest a decay constant in the range of 10⁹ to 10¹² GeV, a range derived from astrophysical limits and cosmological consistency. This range implies that axions could be realized in nature as very light particles that have evaded detection so far.

Furthermore, the paper discusses the potential of axions as candidates for dark matter, a role supported by the coherence in their collective motion despite their extremely low individual masses. This implies a transformative role in understanding mass discrepancies in galaxies and the large-scale structure around us.

Laboratory Constraints and Observational Efforts

Laboratory searches for axions have primarily relied on indirect methods, such as examining solar axions and employing high-intensity magnetic fields (as in axion helioscopes). Efforts like the CERN Axion Solar Telescope (CAST) have significantly constrained the parameter space for axion mass and their photon-coupling strength.

The detection of axionic signals through cavity experiments also provides a novel possibility for directly observing these particles, relying on their possible conversion to microwave photons under controlled conditions. The predictions and constraints on axions outlined in this paper not only shape laboratory searches but also contribute to setting thresholds for future experimental endeavors.

Future Research and Theoretical Models

The supersymmetrization of axion models introduces additional particles such as the axino, which could influence universe evolution significantly. This presents another complex dimension to the already intricate solution to the strong CP problem.

Looking into the future, significant theoretical advancements in string theory and extra-dimensional models may provide further clarity on the natural scale for the axion and integrate them more seamlessly into the fabric of modern particle physics. Theoretical models continue to be refined with improved observational data, forming a feedback loop that consistently narrows down the possible characteristics of axions in nature.

Conclusion

The research encapsulated in the paper claims that solving the strong CP problem elegantly with axions also provides a window into new physics beyond the Standard Model, serving as a critical intersection of particle physics, cosmology, and astrophysics. The paper effectively synthesizes various approaches—cosmological limits, astrophysical dynamics, laboratory constraints—into a comprehensive discussion that solidifies the axion's position in current theoretical physics while also urging for new avenues of exploration as technology evolves.