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PQ Axiverse
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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.
Forward citations
Cited by 10 Pith papers
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Constraining axion quadratic couplings with the Hulse-Taylor binary system
Orbital timing of the Hulse-Taylor binary pulsar gives the strongest published limits on quadratic axion-muon couplings for axion masses below about 10^-12 eV, and complementary limits on axion-neutron and axion-elect...
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Axiverse Lampposts
In a hierarchical multi-axion theory with random couplings, axion field ranges shrink with 1/sqrt(N), generic axion–SM couplings are suppressed, but the QCD axion's coupling is unsuppressed.
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Towards a Heterotic Axiverse
Generic heterotic axions are heavier than in type IIB; the Strong CP problem is typically unsolved, and a light fuzzy dark matter axion requires a no-gaugino-condensation anisotropic fibred Calabi-Yau.
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Universality in the Axiverse
Normalized divisor volumes in toric Calabi-Yau threefolds appear universal, and a GOE level-spacing model with a fitted mean reproduces axion statistics across the Kreuzer-Skarke axiverse.
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Resonant Landau-Zener Conversion In Multi-Axion Systems
The paper derives a Landau-Zener conversion probability for non-adiabatic axion mass-level crossings and uses it to predict two-axion dark matter abundances and haloscope targets.
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Constraining the axiverse with reionization
Roughly 10 to 15 percent of explicit string theory axiverse models are disfavored by CMB reionization data unless reheating stayed below about 10^10 GeV.
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String Theory and Grand Unification Suggest a Sub-Microelectronvolt QCD Axion
String-theoretic axions consistent with grand unification and proton decay are predicted to have masses in the 3e-11 to 1e-8 eV window, with at most 47 axions in the Kreuzer-Skarke ensemble.
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Hierarchical Axiverse
The paper asserts that the requirement of well-defined QCD-induced mixing alone forces ALP masses into a hierarchical spacing and decay constants into two separated populations, but it does not demonstrate the underly...
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Wading the String Bog: CMB Birefringence in the Swampland
Ultralight-axion explanations of cosmic birefringence sit in tension with Swampland bounds, while thin axionic domain walls predict exactly isotropic birefringence, so anisotropy measurements can discriminate the alte...
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Axions at the meV Crossroads: Theory, Cosmology, Astrophysics, and Experiments
The meV axion window is presented as a coherent, cross-validated search program in which string theory, stellar cooling, dark matter, and new detector concepts converge on the same mass range.
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