PQ Axiverse

Abstract

We show that the strong CP problem is solved in a large class of compactifications of string theory. The Peccei-Quinn mechanism solves the strong CP problem if the CP-breaking effects of the ultraviolet completion of gravity and of QCD are small compared to the CP-preserving axion potential generated by low-energy QCD instantons. We characterize both classes of effects. To understand quantum gravitational effects, we consider an ensemble of flux compactifications of type IIB string theory on orientifolds of Calabi-Yau hypersurfaces in the geometric regime, taking a simple model of QCD on D7-branes. We show that the D-brane instanton contribution to the neutron electric dipole moment falls exponentially in $N^4$, with $N$ the number of axions. In particular, this contribution is negligible in all models in our ensemble with $N>17$. We interpret this result as a consequence of large $N$ effects in the geometry that create hierarchies in instanton actions and also suppress the ultraviolet cutoff. We also compute the CP breaking due to high-energy instantons in QCD. In the absence of vectorlike pairs, we find contributions to the neutron electric dipole moment that are not excluded, but that could be accessible to future experiments if the scale of supersymmetry breaking is sufficiently low. The existence of vectorlike pairs can lead to a larger dipole moment. Finally, we show that a significant fraction of models are allowed by standard cosmological and astrophysical constraints.

arXiv comment15 pages, 8 figures, 2 appendices. v2: typo fixed, references added

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• ALP Anarchy
• The problem of flavour
• A QCD R-Axion
• Composite heavy axion-like dark matter
• A Quantal Theory of Restoration of Strong CP Symmetry
• The standard HVM
• Zero Modes of Massive Fermions Delocalize from Axion Strings
• Axion-photon conversion down to the nonrelativistic regime
• A Prototype Hybrid Mode Cavity for Heterodyne Axion Detection
• Axions at the meV Crossroads: Theory, Cosmology, Astrophysics, and Experiments
• Nucleosynthesis and CMB bounds on photophilic ALPs: a fresh look
• Inching toward the QCD Axions with Axion Magnetic Resonance in Helioscopes
• Physics beyond the Standard Model with the DSA-2000