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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 22 Pith papers
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The Holographic QCD Axion in Five Dimensions
A 5D holographic QCD axion model identifies bulk modes for the axion and eta prime, traces the quality problem to insufficient compositeness, and finds the physical axion mostly in the bulk gauge field when quality is high.
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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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Testing F-theory GUTs with the Axiverse
In F-theory GUTs, non-universal ALPs induced by hypercharge flux satisfy g_aγ/m_a well below the QCD axion prediction when gauge couplings unify near the string scale.
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Searches for GeV-Scale ALPs at RHIC
Proposes searching for GeV-scale ALPs in ultra-peripheral collisions at RHIC via γγ → a → γγ, estimating sensitivity to m_a ~2-5 GeV and g_aγγ ≳4e-4 GeV^{-1} with existing PHENIX data.
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How well can the QCD axion hide?
Multi-axion models relax the E/N bound on QCD axion photon coupling and allow subdominant dark matter contribution, but an axion-like particle is typically visible to next-generation experiments.
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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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A Quantal Theory of Restoration of Strong CP Symmetry
A mechanism is proposed where the CP violating phase relaxes in discrete steps via nucleation of membranes that discharge the magnetic dual of a 4-form flux, with bubbles colliding and percolating near the chiral symm...
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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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Quantum Gravity Cutoff from Axions: A Type IIB Landscape Study
The bound Λ_QG ≲ 2π √S f holds for C2 and C4 axions in Type IIB Calabi-Yau compactifications even near Kähler moduli space boundaries where co-scaling fails.
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Counting axions with IAXO
Projections for IAXO in two-axion parameter space plus spectral analysis of flavor oscillations show where the experiment can discriminate multi-axion signals from single-axion ones, extending to N-axion cases.
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Tachyonic Encore: A universal shift of inflationary observables
A light axion spectator induces post-inflation tachyonic phases that produce a nearly scale-invariant boost to the curvature power spectrum and alter key inflationary observables in a largely potential-independent manner.
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The Atacama Cosmology Telescope: Probing new signatures of ultralight axions with gravitational lensing
New upper limits on ultralight axion dark matter fraction (<1.5% at 10^{-26} eV, <9% at 10^{-25} eV at 95% CL) from combined CMB lensing data, plus marginal 2.1σ preference at 10^{-24.5} eV driven by few points.
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Magnetic Turbulence Boosts Supernova Signals of Axion-Photon Conversion
Turbulent magnetic fields enhance axion-photon conversion signals from supernovae, improving limits on axion-proton and axion-photon couplings by up to two orders of magnitude.
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Constraining F-theory Model Building with QCD Axions
QCD axions constrain F-theory base threefolds to have rigid or flux-rigidified divisors, yielding typical axion masses around 10^{-9} eV and decay constants near 10^{15} GeV in allowed regions.
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Constraining F-theory Model Building with QCD Axions
F-theory models with the Standard Model spectrum are constrained by QCD axion physics, yielding typical detectable axion masses around 10^{-9} eV and decay constants around 10^{15} GeV in allowed regions.
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Nucleosynthesis and CMB bounds on photophilic ALPs: a fresh look
Updated model-independent BBN and CMB bounds on photophilic ALPs that incorporate rare decays to light hadrons, show extended constraints for multiple reheating temperatures, and flag parameter space that may alleviat...
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Completing Axion Double Level Crossings
Double level crossings are common in multi-axion mass mixing of Z_N axions and ALPs, occurring multiple times for large N in the light axion case but prevented by excessively large or small N in light and heavy scenar...
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CMB Constraints on Pre-Inflationary Axion Dark Matter Isocurvature
Combined CMB datasets moderately tighten the allowed range for the spectral index of axion isocurvature but leave amplitude bounds for fixed indices unchanged.
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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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