WISPy Cold Dark Matter

TL;DR

The paper demonstrates that very light, weakly interacting bosons produced non-thermally via the misalignment mechanism—specifically axion-like particles and hidden photons—can form the observed cold dark matter across broad photon-coupled parameter spaces. It analyzes the cosmological evolution, including condensate survival, thermal populations, and decay/interaction constraints, and identifies substantial portions of the viable region accessible to current and upcoming searches. The work highlights the complementary reach of haloscopes, helioscopes, and light-shining-through-a-wall experiments in probing WISPy CDM, while outlining key indirect bounds from photon propagation, CMB, and X-ray/optical observations. Overall, the results motivate extensive experimental programs to explore WISPy dark matter and emphasize the rich interplay between theory, cosmology, and laboratory searches.

Abstract

Very weakly interacting slim particles (WISPs), such as axion-like particles (ALPs) or hidden photons (HPs), may be non-thermally produced via the misalignment mechanism in the early universe and survive as a cold dark matter population until today. We find that, both for ALPs and HPs whose dominant interactions with the standard model arise from couplings to photons, a huge region in the parameter spaces spanned by photon coupling and ALP or HP mass can give rise to the observed cold dark matter. Remarkably, a large region of this parameter space coincides with that predicted in well motivated models of fundamental physics. A wide range of experimental searches -- exploiting haloscopes (direct dark matter searches exploiting microwave cavities), helioscopes (searches for solar ALPs or HPs), or light-shining-through-a-wall techniques -- can probe large parts of this parameter space in the foreseeable future.

Figures (5)

• Figure 1: Parameter space for axions (shaded band labelled "Axion models") and axion-like particles. The regions where they could form DM are displayed in different shades of red (for details see text). The lines representing DM regions are uncertain through a model-dependent multiplicative factor, ${\cal N}$, which we have set equal to 1 here. The DM regions move towards larger couplings $g$, proportional to this factor. The exclusion regions labelled "ALPS", "CAST+Sumico" and "HB" arise from experiments and astrophysical observations that do not require ALP dark matter (for a review, see Jaeckel:2010ni). The remaining constraints are based on ALPs being DM and are described in the text.
• Figure 2: Region that would be affected by resonant ALP-photon oscillations leading to the evaporation of the ALP condensate in the case where primordial magnetic fields exist and they evolve with redshift as $B=B_0(1+z)^2$.
• Figure 3: Exclusion bounds on axion-like particles from relic photons in the mass--lifetime parameter space.
• Figure 4: The decay rate of the HP condensate, $\Gamma_{2}$, normalized to $H\chi^2$, as a function of temperature, for different HP masses. The curves, ordered by their resonance from left to right, correspond to ${m_{\gamma'}}=10^{-5},10^{-4},10^{-3},10^{-2}$ and 0.1 eV. The dashed line corresponds to the expression used in Ref. arXiv:1105.2812.
• Figure 5: Allowed parameter space for hidden photon cold dark matter (HP CDM) (for details see text). The exclusion regions labelled "Coulomb", "CMB", "ALPS", "CAST" and "Solar Lifetime" arise from experiments and astrophysical observations that do not require HP dark matter (for a review see Jaeckel:2010ni). We also show constraints on the "cosmology of a thermal HP DM". Note that only constraints on HPs with masses below twice the electron mass are shown since otherwise the cosmological stability condition requires unreasonably small values of the kinetic mixing, $\chi$. The four constraints that bound the allowed region from above, "$\tau_2>$1", "CMB distortions", "$N_\nu^{\rm eff}$" and "X-rays" are described in the text.