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Precise predictions for the QCD axion contribution to dark radiation with full phase-space evolution
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abstract
We compute the QCD axion contribution to the energy density of dark radiation, parameterized by $\Delta N_{\rm eff}$, by solving Boltzmann equations for the momentum distribution functions including the effects of quantum statistics for all particles involved in the axion production processes. This approach gives precise prediction for $\Delta N_{\rm eff}$ independently on whether axions are produced via freeze-out or freeze-in. We focus on axions produced via flavor-conserving and flavor-violating interactions with leptons. Our precise predictions for $\Delta N_{\rm eff}$ can differ from those assuming thermal shape for the momentum distribution functions, as commonly done in the literature, by more than the experimental precision of future Cosmic Microwave Background (CMB) observations. Current lower limits on the axion couplings from Planck constraints on $\Delta N_{\rm eff}$ are also affected by our precise computation which, in particular, results in a strongly relaxed bound on flavor-violating axion couplings to tau lepton and muon or electron.
Forward citations
Cited by 7 Pith papers
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Sensitivity of Next-Generation CMB Surveys to Neutrinos and Other Light Relics
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Beyond thermal approximations: Precise cosmological bounds on Axion-Like Particles
Solving the momentum-dependent Boltzmann equation and propagating the exact non-thermal ALP spectrum into CMB analyses yields 95% limits f_a>1.63e6 GeV (e), 9.41e6 GeV (mu), 8.06e4 GeV (tau), and g_a_gamma<1.98e-8 GeV^-1.
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Improved cosmological constraints on axion-lepton interactions
For axion masses above 0.1 eV, cosmology provides the strongest known limits on axion couplings to muons, taus, and flavor-violating tau channels, excluding decay constants up to 10^8 GeV.
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Ultralight Dark Matter from the Edge of Field Space
A scalar with hard field-space boundaries — a 'wallion' — is a viable ultralight dark-matter candidate whose exponentially small, radiatively stable mass and saturated relic density at large misalignment also suppress...
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Using $\Delta N_{\rm eff}$ to constrain preferred axion model dark matter
In preferred axion models with slow heavy-quark decays, axions produced after decoupling form dark radiation that can exceed Planck's ΔNeff bound, excluding much of the parameter space of models D and E.
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CPon Dark Matter
A light scalar from spontaneous CP violation, the CPon, can be a viable dark matter candidate with a mass between about 10 meV and 1 MeV and a suppression scale above 10^12 GeV.
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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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