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The Strong CP Problem and Axions

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arxiv hep-ph/0607268 v1 pith:4LM3RQKY submitted 2006-07-24 hep-ph

classification hep-ph
keywords axionsveryproblemaxionboundsbrieflyinvisiblelight
verification ladder T0 review T1 audit T2 compute T3 formal
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abstract

I describe how the QCD vacuum structure, necessary to resolve the $U(1)_A$ problem, predicts the presence of a P, T and CP violating term proportional to the vacuum angle $\bar{\theta}$. To agree with experimental bounds, however, this parameter must be very small $(\bar{\theta} \leq 10^{-9}$). After briefly discussing some possible other solutions to this, so-called, strong CP problem, I concentrate on the chiral solution proposed by Peccei and Quinn which has associated with it a light pseudoscalar particle, the axion. I discuss in detail the properties and dynamics of axions, focusing particularly on invisible axion models where axions are very light, very weakly coupled and very long-lived. Astrophysical and cosmological bounds on invisible axions are also briefly touched upon.

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Cited by 12 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score. OpenAlex reports about 353 citations worldwide. Full citation record

  1. Axion Inflation with a Massive Abelian Gauge Field

    astro-ph.CO 2026-07 conditional novelty 6.0 of 10

    A massive Abelian gauge field axially coupled to the inflaton yields tachyonic production only for |ξ|>m/H, with sourced CMB curvature power suppressed as ~1/m̄²–1/m̄³ at fixed backreaction, allowing strong friction f...

  2. MeV Electrophilic Axion-like Particles from Sun

    hep-ph 2026-07 conditional novelty 6.0 of 10

    MeV axion-like particles could be made when 5.5 MeV solar fusion photons Compton-scatter off electrons; current LZ, PandaX-4T and Borexino data would then constrain g_ae to (1.7-3.7)e-6 in the 0.4-1 MeV window.

  3. Gravitational Properties of the Monopole Bag

    hep-ph 2026-04 unverdicted novelty 6.0 of 10

    Monopole bags in axion models can collapse into horizonless objects or dyonic regular black holes that evade singularities and retain axionic structure through Chern-Simons effects.

  4. Searching for axions with time resolved pulsar polarimetry

    hep-ph 2026-03 unverdicted novelty 6.0 of 10

    Time-resolved optical polarimetry of the Crab pulsar is used to place bounds on the axion-photon coupling via oscillating axion fields induced by the pulsar's magnetic field.

  5. Worldline effective field theory of inspiralling black hole binaries in presence of dark photon and axionic dark matter

    hep-th 2023-05 unverdicted novelty 6.0 of 10

    Computes 1PN conservative dynamics for gravitational/EM/Proca fields and 2PN for scalar, plus radiation effects from axion-photon coupling at high PN orders in binary black hole systems with dark matter.

  6. Yukawa-Screened Bose-Star Condensation

    hep-ph 2026-05 unverdicted novelty 5.0 of 10

    Yukawa screening in the YSP system delays Bose-star condensation and broadens the equilibrium density profile, matching a modified kinetic formula after one overall normalization fit.

  7. Constraining F-theory Model Building with QCD Axions

    hep-th 2026-05 unverdicted novelty 5.0 of 10

    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.

  8. Universal lower bound on the axion decay constant from free streaming effects

    astro-ph.CO 2025-07 conditional novelty 5.0 of 10

    Abundance-boosted axion dark matter generically produces warm nonzero-momentum modes, so Lyman-alpha free-streaming and isocurvature bounds exclude fa below about 10^14-10^15 GeV (10^-18 eV/ma) for most ultralight axi...

  9. A multi-axion model of inflation and dark matter

    hep-ph 2026-07 reject novelty 4.0 of 10

    A 300-mode Kaluza-Klein axion tower is proposed as a unified inflaton and dark-matter sector, but the lightest state's quoted lifetime contradicts the paper's own decay-rate formula.

  10. Notes from the bulk

    hep-th 2025-07 conditional novelty 4.0 of 10

    A boundary-QFT analysis shows that curved bulk metrics create local edge velocities in topological models and that fracton and linearized-gravity theories carry Kac-Moody boundary algebras.

  11. Tracing Ultra Light Axions in Post-reionization, Lyman-$\alpha$ and CMB Missions

    astro-ph.CO 2026-06 unverdicted novelty 3.0 of 10

    Fisher forecasts indicate joint post-reionization LSS cross-spectra and CMB-S4-like data can reach O(10^{-4}) uncertainty on ultra-light axion fraction for m_a ≲ 10^{-28} eV, with peak sensitivity near 10^{-25} eV.

  12. The landscape of QCD axion models

    hep-ph 2020-03 unverdicted novelty 2.0 of 10

    Review classifies QCD axion models extending the standard mass-coupling window and updates bounds from cosmology, astrophysics, and experiments.

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