Exploring the Role of Axions and Other WISPs in the Dark Universe

TL;DR

This work argues that very weakly interacting slim particles (WISPs) like the QCD axion, axion-like particles, and hidden photons are well-motivated dark-matter candidates produced non-thermally via vacuum realignment. It connects ultraviolet completions, notably string theory and LVS scenarios, to characteristic ranges of decay constants, masses, and couplings, identifying theoretical hotspots in parameter space. The paper surveys a broad array of astrophysical, cosmological, and laboratory constraints, while outlining a rich experimental program—haloscopes, helioscopes, and light-shining-through-walls—that can probe substantial portions of the WISP landscape and potentially reveal a hidden sector connected to dark matter and early-universe physics. The combined theoretical and experimental narrative highlights the potential for WISPs to illuminate the dark universe and motivate next-generation low-energy, high-intensity searches with broad scientific payoff.

Abstract

Axions and other very weakly interacting slim particles (WISPs) may be non-thermally produced in the early universe and survive as constituents of the dark universe. We describe their theoretical motivation and their phenomenology. A huge region in parameter space spanned by their couplings to photons and their masses can give rise to the observed cold dark matter abundance. A wide range of experiments - direct dark matter searches exploiting microwave cavities, searches for solar axions or WISPs, and light-shining-through-a-wall searches - can probe large parts of this parameter space in the foreseeable future.

Figures (4)

• Figure 1: In compactifications of type II string theories the standard model is locally realised by a stack of space-time filling $D$-branes wrapping cycles in the compact dimensions (figure from Ref. Jaeckel:2010ni). In general, there can also be hidden sectors localised in the bulk. They can arise from branes of different dimension ($D3$ or $D7$ branes) which can be either of large extent or localised at singularities. Light visible and hidden matter particles arise from strings located at intersection loci and stretching between brane stacks.
• Figure 2: Axion and ALP coupling to photons vs. its mass (adapted from Refs. Hewett:2012nsBaker:2012esgCadamuro:2011fdArias:2012mb). Colored regions are: generic prediction for the QCD axion, exploiting Eqs. (\ref{['axionmass']}) and (\ref{['axionphotoncoupling']}), which relate its mass with its coupling to photons (yellow), experimentally excluded regions (dark green), constraints from astronomical observations (gray) or from astrophysical or cosmological arguments (blue), and sensitivity of planned experiments (light green). Shown in red are boundaries where axions and ALPs can account for all the cold dark matter produced either thermally or non-thermally by the vacuum-realignment mechanism.
• Figure 3: A summary of constraints on and hints for the ratio of decay constants and couplings, $f_{a_i}/C_{ij}$, of the axion and axion-like particles $i$ to fields $j$ (from Ref. Cicoli:2012sz). The green regions from top to bottom correspond respectively to the classic 'axion dark matter window', hints of an axion from white dwarf cooling and transparency of the Universe to very high energy gamma rays. Red regions are excluded, and the orange region would be excluded by red giants but is compatible with the hints from white dwarfs. The blue region would be excluded by dark matter overproduction in the absence of a dilution mechanism or tuning of the misalignment angle.
• Figure 4: Kinetic mixing parameter vs. hidden photon mass (adapted from Refs. Arias:2012mbHewett:2012nsBaker:2012esg). Colored regions are: experimentally excluded regions (dark green), constraints from astronomical observations (gray) or from astrophysical or cosmological arguments (blue), and sensitivity of planned experiments (light green). Shown in red are boundaries where the hidden photon would account for all cold dark matter produced either thermally or non-thermally by the vacuum-realignment mechanism or where the hidden photon could account for the hint of dark radiation during the CMB epoch. The regions bounded by dotted lines show predictions from string theory corresponding to different possibilities for the nature of the hidden photon mass: Hidden-Higgs, a Fayet-Iliopoulos term, or the Stückelberg mechanism. In general, predictions are uncertain by factors of order one.