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 nd 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.

/pdf/wispy-cold-dark-matter-208aqdpva0.pdf

WISPy cold dark matter

During 2003--2015, the CERN Axion Solar Telescope (CAST) has searched for $a\to\gamma$ conversion in the 9 T magnetic field of a refurbished LHC test magnet that can be directed toward the Sun. In its final phase of solar axion searches (2013--2015), CAST has returned to evacuated magnet pipes, which is optimal for small axion masses. The absence of a significant signal above background provides a world leading limit of $g_{a\gamma} < 0.66 \times 10^{-10} {\rm GeV}^{-1}$ (95% C.L.) on the axion-photon coupling strength for $m_a \lesssim 0.02$ eV. Compared with the first vacuum phase (2003--2004), the sensitivity was vastly increased with low-background x-ray detectors and a new x-ray telescope. These innovations also serve as pathfinders for a possible next-generation axion helioscope.

/pdf/new-cast-limit-on-the-axion-photon-interaction-4205zokqk8.pdf

New CAST Limit on the Axion-Photon Interaction

Four decades after its prediction, the axion remains the most compelling solution to the strong-CP problem and a well-motivated dark matter candidate, inspiring a host of elegant and ultrasensitive experiments based on axion–photon mixing. This article reviews the experimental situation on several fronts. The microwave cavity experiment is making excellent progress in the search for dark matter axions in the μeV range and may plausibly be extended up to 100 μeV. Within the past several years, however, researchers have realized that axions are pervasive throughout string theories, but with masses that fall naturally in the neV range, for which an NMR-based search is under development. Both searches for axions emitted from the Sun's burning core and purely laboratory experiments based on photon regeneration have recently made great progress, with ambitious projects proposed for the coming decade. Each of these campaigns has pushed the state of the art in technology, enabling large gains in sensitivity and m...

https://www.annualreviews.org/doi/pdf/10.1146/annurev-nucl-102014-022120

Experimental Searches for the Axion and Axion-Like Particles

New ALPS results on hidden-sector lightweights

Axion-like particles (ALPs), which are gauge-singlets under the Standard Model (SM), appear in many well-motivated extensions of the SM. Describing the interactions of ALPs with SM fields by means of an effective Lagrangian, we discuss ALP decays into SM particles at one-loop order, including for the first time a calculation of the a → πππ decay rates for ALP masses below a few GeV. We argue that, if the ALP couples to at least some SM particles with couplings of order (0.01 − 1) TeV−1, its mass must be above 1 MeV. Taking into account the possibility of a macroscopic ALP decay length, we show that large regions of so far unconstrained parameter space can be explored by searches for the exotic, on-shell Higgs and Z decays h → Za, h → aa and Z → γa in Run-2 of the LHC with an integrated luminosity of 300 fb−1. This includes the parameter space in which ALPs can explain the anomalous magnetic moment of the muon. Considering subsequent ALP decays into photons and charged leptons, we show that the LHC provides unprecedented sensitivity to the ALP-photon and ALP-lepton couplings in the mass region above a few MeV, even if the relevant ALP couplings are loop suppressed and the a → γγ and a → l+l− branching ratios are significantly less than 1. We also discuss constraints on the ALP parameter space from electroweak precision tests.

/pdf/collider-probes-of-axion-like-particles-1je1gfzke8.pdf

Collider probes of axion-like particles

We have searched for solar axions or other pseudoscalar particles that couple to two photons by using the CERN Axion Solar Telescope (CAST) setup. Whereas we previously have reported results from CAST with evacuated magnet bores (Phase I), setting limits on lower mass axions, here we report results from CAST where the magnet bores were filled with 4He gas (Phase II) of variable pressure. The introduction of gas generates a refractive photon mass mγ, thereby achieving the maximum possible conversion rate for those axion masses ma that match mγ. With 160 different pressure settings we have scanned ma up to about 0.4 eV, taking approximately 2 h of data for each setting. From the absence of excess x-rays when the magnet was pointing to the Sun, we set a typical upper limit on the axion-photon coupling of gaγ2.2 × 10−10 GeV−1 at 95% CL for ma0.4 eV, the exact result depending on the pressure setting. The excluded parameter range covers realistic axion models with a Peccei-Quinn scale in the neighborhood of fa ~ 107 GeV. Currently in the second part of CAST Phase II, we are searching for axions with masses up to about 1.2 eV using 3He as a buffer gas.

/pdf/probing-ev-scale-axions-with-cast-2xnu1312t9.pdf

E. Arik

Papers

Probing eV-scale axions with CAST