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Axions and the Strong CP Problem

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arxiv 0807.3125 v2 pith:WN5WOE6M submitted 2008-07-20 hep-ph astro-phhep-exhep-th

Axions and the Strong CP Problem

classification hep-ph astro-phhep-exhep-th
keywords axionproblemthetadatalesssimstrongangleastrophysical
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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Current upper bounds of the neutron electric dipole moment constrain the physically observable quantum chromodynamic (QCD) vacuum angle $|\bar\theta| \lesssim 10^{-11}$. Since QCD explains vast experimental data from the 100 MeV scale to the TeV scale, it is better to explain this smallness of $|\bar\theta|$ in the QCD framework, which is the strong \Ca\Pa problem. Now, there exist two plausible solutions to this problem, one of which leads to the existence of the very light axion. The axion decay constant window, $10^9\ {\gev}\lesssim F_a\lesssim 10^{12} \gev$ for a ${\cal O}(1)$ initial misalignment angle $\theta_1$, has been obtained by astrophysical and cosmological data. For $F_a\gtrsim 10^{12}$ GeV with $\theta_1<{\cal O}(1)$, axions may constitute a significant fraction of dark matter of the universe. The supersymmetrized axion solution of the strong \Ca\Pa problem introduces its superpartner the axino which might have affected the universe evolution significantly. Here, we review the very light axion (theory, supersymmetrization, and models) with the most recent particle, astrophysical and cosmological data, and present prospects for its discovery.

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Cited by 23 Pith papers

Reviewed papers in the Pith corpus that reference this work. Sorted by Pith novelty score.

  1. Resonant heterodyne conversion applied to a low-frequency haloscope for dark matter axion searches in the 1-35 MHz range

    hep-ex 2026-06 unverdicted novelty 7.0

    Resonant heterodyne conversion is derived and applied to a two-port haloscope cavity to enable axion searches from 0.9 to 34.6 MHz with projected reach to g_{aγ} = 10^{-15} GeV^{-1}.

  2. An ultra-broadband axion dark matter experiment

    hep-ph 2026-05 conditional novelty 7.0

    A dc SQUID operated at the flux sweet spot with lock-in modulation yields an ultra-broadband axion search with projected sensitivity |g_aγγ| ≳ 10^{-16} GeV^{-1} across 15 orders of magnitude in mass.

  3. Lights, Camera, Axion: Tracing Axions from Supernovae in the Diffuse $\gamma$-ray Sky

    hep-ph 2026-04 unverdicted novelty 7.0

    Axions produced in supernovae generate a diffuse gamma-ray signal through conversion in magnetic fields, yielding competitive constraints on the axion-photon coupling from COMPTEL, EGRET, and Fermi-LAT data plus forec...

  4. Is the Conventional Picture of Coherence Time Complete? Dark Matter Recoherence

    hep-ph 2026-01 unverdicted novelty 7.0

    Ultralight dark matter exhibits recoherence due to the solar gravitational potential, yielding formally divergent coherence times at long timescales and enhanced search sensitivity.

  5. The $B^+ \to K^+ \nu \bar \nu$ decay as a search for the QCD axion

    hep-ph 2025-10 unverdicted novelty 7.0

    Model-independent reinterpretation of Belle II B+ → K+ νν̄ data using public kinematic mapping yields a factor-of-nine improvement on the limit for B+ → K+ a and constrains the axion's b-s flavor-changing coupling.

  6. Relativistic frequency shifts in gravitational waves from axion clouds

    gr-qc 2026-04 unverdicted novelty 6.0

    A unified relativistic framework using bilinear perturbation theory calculates frequency shifts in GWs from axion clouds, handling self-interactions and multiple superradiant modes for the first time.

  7. Searching for axions with quantum interferometry

    hep-ph 2026-04 unverdicted novelty 6.0

    Axion-photon coupling imprints measurable Aharonov-Bohm and Berry phases in superconducting circuits and interferometers, projecting sensitivity to g_aγγ ~ 7.8e-14 GeV^{-1} at m_a ~ 1e-10 eV.

  8. Searching for axions with time resolved pulsar polarimetry

    hep-ph 2026-03 unverdicted novelty 6.0

    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.

  9. Highly Excited Electron Cyclotron for QCD Axion and Dark-Photon Detection

    hep-ph 2024-10 unverdicted novelty 6.0

    Proposes resonant detection of QCD axions (0.1-2.3 meV) and dark photons (down to epsilon ~2e-16) via highly excited electron cyclotron states in an open-endcap Penning trap compatible with large cavities.

  10. Reappraisal of the Constraints on Heavy Axion-like Particles from Gamma-Ray Bursts

    hep-ph 2026-07 conditional novelty 5.0

    Realistic GRB parameters weaken previous ALP cooling bounds, but ALP-induced secondary fireballs in GRBs could still be probed via isotropic X-ray emission from future telescopes.

  11. Axions Create Singularities on Extremal Horizons

    hep-th 2026-05 unverdicted novelty 5.0

    Axions cause singularities on the horizons of extremal rotating charged black holes for almost all axion mass and coupling values.

  12. Constraining F-theory Model Building with QCD Axions

    hep-th 2026-05 unverdicted novelty 5.0

    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.

  13. Constraining F-theory Model Building with QCD Axions

    hep-th 2026-05 unverdicted novelty 5.0

    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.

  14. Axion Quality in Warped Extra-Dimension

    hep-ph 2026-04 unverdicted novelty 5.0

    Warped extra-dimensional axion models achieve high quality when nonlocal U(1)-charged field effects are sufficiently suppressed by the warp factor and orbifold structure.

  15. Detectability of axion-like dark matter for different time-delay interferometry combinations in space-based gravitational wave detectors

    gr-qc 2025-11 unverdicted novelty 5.0

    Monitor and Beacon TDI combinations reach g_aγ ~ 10^{-13} GeV^{-1} at high frequencies while Sagnac performs better at low frequencies, with ASTROD-GW probing axion-like dark matter masses down to 10^{-20} eV.

  16. Serendipitous supersymmetric solution to the strong CP problem

    hep-ph 2026-07 conditional novelty 4.5

    Anomaly-free discrete R-symmetries plus Kim-Nilles regeneration of the mu term force an accidental U(1)_PQ whose spontaneous breaking yields a SUSY DFSZ axion that solves strong CP.

  17. Hidden-sectors search and probe of discrete symmetries at the REDTOP experiment

    hep-ex 2026-06 unverdicted novelty 4.0

    REDTOP targets O(10^14) eta and O(10^12) eta-prime decays to probe four hidden sector portals and test CP/T invariance plus lepton universality.

  18. Shedding Stray Light on Decaying Light Dark Matter: Constraints from NuSTAR X-ray Observations

    hep-ph 2026-06 unverdicted novelty 4.0

    NuSTAR stray-light data yields the strongest indirect bounds on decaying electrophilic scalar, ALP, and dark photon DM in the 6-70 keV range, plus inelastic DM with mass splittings 3-100 keV.

  19. Looking for Lights from the Darkness: Signals from MeV-scale Solar Axion-like Particles

    hep-ph 2026-04 unverdicted novelty 4.0

    Solar axion-like particles up to 5.5 MeV produce off-axis MeV photons via two-body decay, enabling new space and terrestrial searches that could probe g_aγ down to 10^{-12} GeV^{-1}.

  20. Cornering MeV-GeV Axions and Dark Photons with LDMX

    hep-ph 2026-04 unverdicted novelty 4.0

    LDMX can close much of the sub-100 MeV blind spot for axions and dark photons via charged track and momentum reconstruction.

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

    astro-ph.CO 2026-06 unverdicted novelty 3.0

    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.

  22. Solar Axions from Nuclear Transitions

    hep-ph 2026-05 unverdicted novelty 3.0

    Solar axion fluxes from 57Fe and 83Kr nuclear transitions differ by three orders of magnitude, yielding stronger constraints on |g_aN^eff × g_aγγ| and g_aγγ vs. m_a from Chandrayaan-2 XSM data for iron.

  23. Axions as Dark Matter, Dark Energy, and Dark Radiation

    hep-ph 2025-09 unverdicted novelty 2.0

    A mini-review of axion phenomenology showing how light bosons can account for dark matter, drive cosmic acceleration, or contribute to relativistic backgrounds in the early and late Universe.