New CAST limit on the axion–photon interaction

Abstract

Hypothetical low-mass particles, such as axions, provide a compelling explanation for the dark matter in the universe. Such particles are expected to emerge abundantly from the hot interior of stars. To test this prediction, the CERN Axion Solar Telescope (CAST) uses a 9 T refurbished Large Hadron Collider test magnet directed towards the Sun. In the strong magnetic field, solar axions can be converted to X-ray photons which can be recorded by X-ray detectors. In the 2013-2015 run, thanks to low-background detectors and a new X-ray telescope, the signal-to-noise ratio was increased by about a factor of three. Here, we report the best limit on the axion-photon coupling strength (0.66 × 10 −10 GeV −1 at 95% confidence level) set by CAST, which now reaches similar levels to the most restrictive astrophysical bounds.A dvancing the low-energy frontier is a key endeavour in the worldwide quest for particle physics beyond the standard model and in the effort to identify dark matter 1,2 . Nearly massless pseudoscalar bosons, often generically called axions, are particularly promising because they appear in many extensions of the standard model. They can be dark matter in the form of classical field oscillations that were excited in the early universe, notably by the re-alignment mechanism 3 . One particularly well motivated case is the quantum chromodynamics (QCD) axion, the eponym for all such particles. The existence of this new lowmass boson follows from the Peccei-Quinn mechanism as an explanation why QCD is perfectly time-reversal invariant within current experimental precision 3 .Axions were often termed 'invisible' because of their extremely feeble interactions, yet they are the target of a fast-growing international landscape of experiments. Numerous existing and foreseen projects assume that axions are the galactic dark matter and use a variety of techniques that are sensitive to different interaction channels and optimal in different mass ranges 4 . Independently of the dark-matter assumption, one can search for new forces mediated by these low-mass bosons 5 or the back-reaction on spinning black holes (superradiance) 6 . Stellar energy-loss arguments provide restrictive limits that can guide experimental efforts, and in some cases may even suggest new loss channels 3,7,8 .The least model-dependent search strategies use the production and detection of axions and similar particles by their generic twophoton coupling. It is given by the vertex L aγ = −(1/4) g aγ F µν F µν a = g aγ E · B a, where a is the axion field, F the electromagnetic field-strength tensor, and g aγ a coupling constant of dimension (energy) −1 . Notice that we use natural units with = c = k B = 1. This vertex enables the decay a → γ γ , the Primakoff production in stars-that is, the γ → a scattering in the Coulomb fields of charged particles in the stellar plasma-and the coherent conversion a ↔ γ in laboratory or astrophysical B-fields 9,10 .The helioscope concept, in particular, uses a dipole magnet directed at the Sun to convert axions to...

First axion dark matter search with toroidal geometry

“…by this experiment together with previous results from ALPS [21] and CAST [22]. No limits are set from 6.77 to 6.80 GHz due to with a quasi-TE mode in that frequency region and the TE mode is also confirmed by a simulation [19].…”

“…On top of a T p,cavity of 0.1 K, quantum-noise-limited superconducting amplifiers were employed as the 1st amplifiers whose noise temperatures are proportional to not only the frequencies (T n,amp ∼ hν/k B ln 2) [28] but also the ambient temperatures (T p,cavity in axion haloscopes) to achieve fast DFSZ scanning rates from the small AC-TION experiment. Figure 5 shows the exclusion limits expected from the large and small ACTION experiments, and therein the exclusion limits from RBF [12], UF [13], ADMX [14,15], HAYSTAC [16], and CAST [22] are also shown.…”

“…The simplified ACTION experiment excludes the axion-photon coupling g aγγ down to [12], UF [13], ADMX [14,15], HAYSTAC [16], and CAST [22] are also shown.…”

The minimal flavour violating axion

“…Astrophysical, cosmological and low-energy terrestrial data provides the strongest bounds on axion couplings, those to photons (the latest constraints have been recently published in ref. [152]): the upper bounds on the effective couplings can be summed up as [154,155] |g aγγ | 7 × 10 −11 GeV −1 for m a 10 meV Notice that the bounds for masses between 10 eV and 0.1 GeV, which include the so-called MeV window, come from (model dependent) cosmological data [144]. On the other side, for masses larger than the TeV, no constraint is present.…”

New mechanism producing axions in the AQN model and how the CAST can discover them

“…These axions are ultrarelativistic as the typical average energy of the axions emitted by the Sun is hEi ¼ 4.2 keV, see [20]. Today the most sensitive broadband searches for solar axions come from the helioscope CAST at CERN [20].…”