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The Irreducible Axion Background
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abstract
Searches for dark matter decaying into photons constrain its lifetime to be many orders of magnitude larger than the age of the Universe. A corollary statement is that the abundance of any particle that can decay into photons over cosmological timescales is constrained to be much smaller than the cold dark-matter density. We show that an $\textit{irreducible}$ freeze-in contribution to the relic density of axions is in violation of that statement in a large portion of the parameter space. This allows us to set stringent constraints on axions in the mass range $100\rm \;eV - 100\; MeV$. At $10\rm \; keV$ our constraint on a photophilic axion is $g_{a\gamma \gamma} \lesssim 8.1 \times 10^{-14}~{\rm GeV}^{-1}$, almost three orders of magnitude stronger than the bounds established using horizontal branch stars; at $100~{\rm keV}$ our constraint on a photophobic axion coupled to electrons is $g_{aee} \lesssim 8.0 \times 10^{-15}$, almost four orders of magnitude stronger than present results. Although we focus on axions, our argument is more general and can be extended to, for instance, sterile neutrinos.
Forward citations
Cited by 14 Pith papers
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Fuzzy Axions and Associated Relics
In explicit string theory compactifications, fuzzy axion dark matter is rare and typically requires fine-tuning of initial axion displacements or a non-standard reheating epoch to avoid overproducing heavier axions.
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The Dark Dimension meets the Axiverse
Dark dimension dark matter can evade decay constraints if N≳50 axion KK towers share the tower energy, diluting Standard Model energy injection by roughly 1/N.
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Darkly Charged ALPs
Darkly charged ALPs (DALPs) forbid all d=5 ALP-SM operators and admit only two leading d=6 operators (Higgs portal and hypercharge portal), with distinct collider, cooling, and dark-matter phenomenology.
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Heavy neutral leptons from light scalar in fixed target and forward search experiments
SHiP and FPF@FCC-hh are projected to extend experimental sensitivity to a light scalar mixing with the Higgs and to long-lived heavy neutrinos in a B-L seesaw model, with SHiP strongest for scalar masses around 0.5-1 GeV.
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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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Loop-Level Lepton Flavor Violation and Diphoton Signals in the Minimal Left-Right Symmetric Model
Recasting axion limits onto the one-loop H3 couplings of the minimal left-right symmetric model excludes the right-handed scale up to 2×10^9 GeV and could eventually probe 6×10^11 GeV.
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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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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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Time-delayed gamma-ray signatures of heavy axions from core-collapse supernovae
No gamma-ray excess from four supernovae in Fermi-LAT data yields the strongest constraints yet on MeV-to-GeV heavy axions coupled to both QED and QCD.
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Strong Constraints on Dark Photon and Scalar Dark Matter Decay from INTEGRAL and AMS-02 data
Using INTEGRAL x-ray and AMS-02 positron data, the authors set lower limits on dark photon and scalar dark matter decay lifetimes from 10^25 to 10^29 seconds across 1 MeV to 2 TeV masses.
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Probing the Cosmic Axion Background via Axion-Photon Conversion in Filaments
Axion-photon conversion in cosmic filaments would turn dark-matter decay into a gamma-ray background, and the calculated flux excludes axion masses below ~10^-5 eV for GeV–TeV dark matter with lifetimes below ~10^30 s.
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Neutrino lines and photon continua from cascade dark matter decay
In cascade dark-matter decay, neutrino-line searches can beat gamma-ray limits whenever the intermediate mediator is long-lived enough to suppress the photon flux.
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NuSTAR bounds on radiatively decaying particles from M82
NuSTAR's non-detection of an X-ray halo from decaying axions around M82 excludes a previously allowed window of axion-photon coupling for 30-500 keV axion masses.
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Glueball Axion-Like Particles
A pseudoscalar glueball from a dark Yang-Mills sector, called a GALP, is shown to behave like an axion-like particle and to be a viable dark matter candidate with a predicted mass-coupling relation.
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