Electric-magnetic duality invariance implications for axion physics

Abstract

Axions and axion-like particles are very well-motivated candidates for physics beyond the Standard model, which can be probed by multiple existing and projected experiments and astrophysical observations. Theoretical determination of the relevant low energy axion interactions is essential for guiding the corresponding experimental and observational efforts. In this thesis, we revisit the previous theoretical investigations in this direction. In particular, we show that, contrary to assertions in the literature, the main contribution to the axion-photon coupling need not be quantized in units proportional to $e^2$. We discuss a loophole in the argument for this quantization and then provide explicit counterexamples. Based on this, we construct a generic axion-photon effective Lagrangian and find that the axion-photon coupling may be dominated by previously unknown Wilson coefficients. We show that this result implies a significant modification of conventional axion electrodynamics and sets new targets for axion experiments. We find that the electromagnetic interactions of axions can violate the CP symmetry and that future experiments could be sensitive to the corresponding coupling. At the core of our theoretical analysis lies a critical reexamination of the interactions between axions and magnetic monopoles. We develop the effective field theory approach to the Zwanziger theory of quantum electromagnetodynamics and show that, contrary to claims in the literature, magnetic monopoles need not give mass to axions. Moreover, we find that a future detection of an axion or axion-like particle with certain parameters can serve as evidence for the existence of magnetically charged matter.

Besides studying the structure of the low energy axion interactions in the effective field theory approach, we explicitly construct new theoretical models for the axion which realize the newly found interactions. In these models, the PQ mechanism is realized through a coupling of the Peccei-Quinn complex scalar field to magnetically charged fermions at high energies. We consider both the cases of Abelian and non-Abelian magnetic charges. We show that these models indeed solve the strong CP problem and then integrate out heavy magnetic monopoles using the Schwinger proper time method. We find that the models discussed yield axion couplings to the Standard Model which are drastically different from the ones calculated within the KSVZ/DFSZ-type models. As a consequence, large part of the corresponding parameter space can be probed by various projected experiments. Moreover, the axion we introduce is consistent with the astrophysical hints for axions suggested both by the anomalous TeV-transparency of the Universe and by the excessive cooling of horizontal branch stars in globular clusters. We argue that the leading term for the cosmic axion abundance is not changed compared to the conventional pre-inflationary scenario for an axion decay constant $f_a > 10^{12}~\text{GeV}$.