Detection of solar axions

If axions exist, they would be produced in large quantities in the solar interior. Photons from the solar plasma would convert into axions in the Coulomb fields of charged particles via the Primakoff axion-photon conversion. Once produced, axions get out of the star unimpeded and travel to the Earth, offering a great opportunity for direct detection in terrestrial axion helioscopes.

The emission of axions by the Sun is a robust prediction of axion theory. The Primakoff solar axion flux depends only on the axion-photon coupling, a generic property of axion models. The energy distribution of solar axions (1 to 10 keV approx.) reflects the photon energies at the solar core and thus is entirely determined by well-known solar physics. It is independent on the value of the axion mass (as long as it is below ~keV).  If the axion couples to electrons, additional axion production channels may become relevant. But the Primakoff component can be considered as a guaranteed minimum whose search allows, at the least, to place a robust bound on the axion-photon coupling over a wide range of mass values. Solar axions offers a detection strategy that does not rely on cosmological (e.g, the axion being the dark matter) assumptions, and thus is highly complementary to dark matter axion searches.

The axion helioscope concept was proposed by Pierre Sikivie more than 30 years ago. It invokes the conversion of the solar axions back to photons in a strong laboratory magnet. The resulting photons are x-rays that can be detected behind the magnet when it is pointing to the Sun. This has been the strategy followed by the CERN Axion Solar Telescope (CAST) using a decommissioned LHC test magnet that provides a 9 T field inside the two 10 m long, 5 cm diameter, magnet bores. CAST magnet is placed over a platform that allows to move it to track the Sun during 3 hours per day. CAST performance has mostly relied on the availability of this first-class magnet. However, CAST has introduced new elements beyond the conventional helioscope concept, like the focusing the x-rays to increase signal-to-noise ratio, or the use of low background techniques (radiopurity, shielding, offline discrimination) for the detectors. During more than a decade of operation, CAST has put progressively stronger limits on the axion-photon coupling across a wide range of axion masses, in particular being the first helioscope probing relevant QCD axion models of sub-eV mass. The latest of these results, published last year in Nature Physics, sets an upper bound on the axion-photon coupling of 0.66 x 10^-10 GeV^-10 (see Figure 1). This value competes with the strongest bound coming from astrophysics. Advancing beyond this state-of-the-art is now highly motivated, not only because it would mean to venture into regions of parameter space allowed by astrophysics, but also because a number of astrophysical observations seem to hint at precisely this range of parameters. CAST has also searched for solar axions produced via the axion-electron coupling, although the very stringent astrophysical bound on this coupling remains so far unchallenged by experiments.

Figure 1 - Parameter space of the axion and ALPs with the region currently excluded by CAST (blue). Semi-transparent blue regions are sensitivity projection of BabyIAXO and IAXO (both nominal and the upgraded IAXO+). The BabyIAXO prospects are estimated very conservatively using the minimal configuration for the magnet and detectors, and 1 year of data, without considering a possible gas phase. The IAXO+ line represents an extra factor 10 in figure-of-merit with respect the nominal IAXO parameters. Also shown are the sensitivity prospects of the ALPS-II experiment, and various regions related with the Universe transparency to high energy photons. The yellow band correspond to the standard place for QCD axion models.

To substantially improve the current CAST bounds and go deeper into unexplored axion parameter space requires a completely new infrastructure, like the one proposed by the International Axion Observatory (IAXO). IAXO is a next generation axion helioscope aiming at a 10⁴ better signal-to-noise ratio compared to CAST. This means a sensitivity to the axion-photon coupling more than one order of magnitude beyond the CAST bound, deep into unexplored ALP space and in particular probing QCD axion models down to the meV scale (see Figure 1). For axions coupling with electrons, IAXO will be able to be sensitive to values beyond current astrophysical bounds for the first time for any experiment, and directly test the range of values that are hinted if the anomalous cooling observed in white stars is due to axions.

Figure 2.- Conceptual design of IAXO in its baseline configuration. Shown are the magnet cryostat, the eight telescope+detector systems, the service units and the inclination system and rotating platform.

This is only possible by building a new dedicated large-scale magnet, designed maximizing the helioscope figure of merit. The IAXO magnet, shown in Figure 2, will be a superconducting magnet following a large multi-bore toroidal configuration, to efficiently produce an intense magnetic field over a large volume. The design is inspired by the ATLAS barrel and end-cap toroids, the largest superconducting toroids ever built and presently in operation at CERN. Indeed the experience of CERN in the design, construction and operation of large superconducting magnets is crucial for the project.

IAXO will also make extensive use of novel detection concepts pioneered at a small scale in CAST, like x-ray focusing and low background detectors. The former relies on the fact that, at grazing incident angles, it is possible to make x-ray mirrors with high reflectivity. True-imaging optics are possible employing two conic-shaped mirrors, a design principle that is being used in x-ray astronomy for nearly 50 years to build x-ray telescopes. This class of optics -commonly referred to as Wolter-I optics- allows “nesting” or placing concentric, co-focal x-ray mirrors inside one another to achieve high throughput, a principle much like a Russian nesting doll. IAXO envisions optics similar to those used on NASA’s NuSTAR (3-79 keV), an x-ray astrophysics satellite currently in orbit, but optimized for the solar axion spectrum energies. The NuSTAR telescopes consist of multiple mirror pieces that have been thermally-formed or “slumped” to match the desired shape and covered with a multilayer coating to enhance the reflections of x-rays. The coatings consist of alternating layers of a reflector material like platinum and a spacer material like carbon with a thickness of a few to a few tens of atoms per layer and will be optimized for the IAXO’s axion search. Each of the eight magnet bores will be equipped with such optics.

At the focal plane in each of the optics, IAXO will have low-background x-ray detectors. Several technologies are under consideration, but the most developed one are small gaseous chambers read by pixelised microbulk Micromegas planes. These detectors have been extensively developed in the recent years in the context of rare event experiments, and in CAST in particular. They involve low-background techniques typically developed in underground laboratories, like the use of radiopure detector components, appropriate shielding, and the use of offline discrimination algorithms, but adapted to the particularities of operation at surface. Micromegas detectors have proven to be largely amenable to these developments, due to the relative simplicity of their design (for a highly pixelised detector, and correspondingly high number of readout channels). The core of these detectors, the microbulk readout, is of special interest as it is manufactured out of raw foils made of kapton and copper, two materials known to be very radiopure. Alternative or additional x-ray detection technologies are also considered for IAXO, like GridPix detectors, Magnetic Metallic Calorimeters, Transition Edge Sensors, or Silicon Drift Detectors. All of them show promise to outperform the baseline Micromegas detectors in aspects like energy threshold or resolution, of interest e.g. to search for solar axions via the axion-electron coupling (and featuring both lower energies that the standard Primakoff ones, as well as monochromatic peaks in the spectrum).

Although both x-ray focusing and low background techniques were exploited since an early phase of the CAST experiment, only in the most recent data taking run were both combined in the same detection system. Figure 3 shows the “IAXO pathfinder” system that took data in one CAST bore in 2014-15 and that enabled the latest result of the experiment above mentioned. It was so dubbed because it includes 1) the first purpose-built x-ray optics for axions, implementing (at a much lower scale) the same technique proposed for IAXO, and 2) the most advanced shielded Micromegas x-ray detector so far, placed at the focal point. The system registered a record background level of an average of 0.003 counts/hour (i.e. an average of one count in the signal region during 6 months of operation of CAST). Figure 4 shows and explains some representative data plots from this detector system.

Figure 3 - Picture of the IAXO pathfinder system (with the detector shielding removed) as installed in one of the magnet bores of CAST, and composed of an x-ray optics and a Micromegas detector placed at the focal spot of the optics.

IAXO is currently in the technical design phase. Important recent milestones include the formalization of the international collaboration last year, currently encompassing 17 institutions from all over the world, including CERN and DESY, the latter likely to host the experiment. The project received good recommendation from the CERN SPSC in an early stage and is now is being reviewed as part of the Physics Beyond Colliders (PBC) Study. As mentioned above, CERN unique experience in magnet technologies is very important for the experiment. Effective CERN support in this regard is already being materialized in the magnet design efforts, in particular via the PBC technology subgroup. The collaboration is now working to realize BabyIAXO, an intermediate-scale version of the full experiment, that will serve to prototype (and potentially improve) all subsystems of the experiment and mitigate risks for the full infrastructure, but at the same time will operate as a fully-fledged axion helioscope. BabyIAXO magnet will feature two bores, and therefore two detection system, but of representative dimensions of the full IAXO systems. As shown in Figure 1, BabyIAXO will enjoy relevant physics case in itself, already with potential for discovery. It is expected that it could be built in 2-3 years and bring the solar axion community into physics production relatively early, while preparing the ground for the full IAXO. The fact that BabyIAXO has attracted ERC funding in this year’s call is a proof of the excitement of these prospects and adds credibility to the near term plans of the project.

Figure 4.- 2D hitmaps of the Micromegas detector of the IAXO pathfinder system at CAST, durign a calibration run (left) and the integrated 6.5 months data run in axion-sensitive “sun tracking” conditions (right). The calibration is performed with an X-ray source placed ∼12 m away (at the other side of the magnet). The contours in the calibration run represent the 95%, 85% and 68% signal-encircling regions from ray-trace simulations, taking into account the source size and distance. In the sun tracking plot, grey dots represent events that pass all detector cuts but that are in coincidence with the muon vetoes, and therefore rejected. Black dots represent final counts. Closed contours indicate the 99%, 95%, 85% and 68% signal-encircling regions out of detailed ray-trace simulations of the telescope plus spatial resolution of the detector. The large dashed circle represents the region of detector exposed to daily energy calibration.

Detection of dark matter axions

If our galactic dark matter halo is made of axions, we would be embedded in an “axion sea” of huge number density. These axions are non-relativistic and with velocities much smaller that their mass, so they are best viewed as a scalar classical field oscillating with a fixed frequency equal to the axion mass. Despite the feeble interaction these axions can be detected exploiting coherence effects. The conventional axion haloscope concept proposed by Sikivie, invokes the conversion of these axions into photons inside a magnet and their resonant detection in a microwave cavity. The signal power is proportional to the quality factor Q of the cavity, although this amplification works only in a narrow relative frequency window of 1/Q around the axion mass.  Given that the axion mass is unknown, these experiments must allow tuning the resonant frequency over a range as wide as possible. Data taking with haloscopes thus entails scanning very thin mass-slices of parameter space.

The ADMX experiment at the U. of Washington is the most advanced implementation of this concept. After more than 25 years of R&D, ADMX has proven that the concept can realistically achieve sensitivity to QCD axions in the few μeV mass ballpark. If the axion mass is in the μeV range and the DM is made mostly of axions, ADMX will find it in the coming years. However, there is a strong motivation to perform axion DM searches at higher masses too. Applying the haloscope technique at higher masses is challenging, because higher frequencies imply lower cavity volumes and correspondingly lower signals. This drop in sensitivity may be compensated with improvements in other experimental parameters (higher magnetic field, lower noise, higher quality factors), like the recent results from the HAYSTAC collaboration at 24 μeV shows. But going to even larger masses will probably require other approaches. During the last years, several ideas have been proposed and many new groups are starting R&D and prototyping work to tackle this challenge. An appealing approach is to somehow equip a large detection volume with a high-frequency resonant structure, in other words, to decouple resonant frequency (and axion mass) from cavity volume. Two possible ways to implement this strategy are currently being tested at CAST at CERN.

After the completion of the original solar axion program of CAST in 2015, the collaboration turned its physics goal to the search for solar chameleons and DM axions. The latter is represented by two subprojects, CAST-CAPP and RADES, both aiming at using the CAST magnet to develop effective high-mass axion haloscopes.

The Relic Axion Detector Exploratory Setup (RADES) aims at developing long-aspect-ratio cavities (that can fit in the CAST dipole) by physically appending many smaller rectangular cavities, interconnected with irises, in what resembles a RF filter structure. The precise geometry of the cavity can be optimized to obtain maximal coupling to the axion field for a given resonant mode, or alternatively to simultaneously share it among several modes at different frequencies, which opens interesting possibilities. This approach allows to (in principle, arbitrarily) enlarge the volume of the overall cavity but keeping a high resonant frequency, which is determined by the size of the unit cavity. The need for external phase matching is avoided, as it is guaranteed by design, and the concept offers a much better control of mode crowding. The concept has been successfully proven with small scale fixed-frequency 8.4 GHz (about 35 μeV) of only 5 unit-cavities shown in Figure 5, that has been already tested inside the CAST magnet. The next steps include the implementation of a mechanism of frequency tuning, and the construction of prototypes with progressively larger number of unit-cavities. Effectively filling the whole bore of the CAST magnet with such structure should provide relevant sensitivity to QCD DM axions in the mentioned mass value.

Figure 5 - First RADES prototype. Left: Cavity body in stainless steel before coating and assembly. Right: copper-coated cavity mounted onto the stick used to insert it inside the CAST magnet.

The installation of the CAST-CAPP/IBS was also completed earlier this summer. These cavities were developed in strong collaboration with the CAPP centre in Korea, following the centre's international expertise in axion searches. CAST-CAPP/IBS Detector project is a haloscope search for axion DM with rectangular tunable cavities inserted in the bores of the CAST dipole magnet.

Following preliminary cavity engineering models, the first fully tunable cavities were produced in Korea and installed in CAST this summer. Their sensitivity could reach into the QCD axion parameter space over the unexplored region of (2–3)×10⁻⁵ eV axion mass range/ First results are expected in the next weeks allowing to understand the behaviour of these cavities before exploiting their full potential.