International Axion Observatory

The International Axion Observatory (IAXO) is a next-generation axion helioscope for the search of solar axions and axion-like particles. It is the follow-up of the CERN Axion Solar Telescope (CAST), which operated from 2003 to 2022. IAXO will be set up by implementing the helioscope concept bringing it to a larger size and longer observation times.

The IAXO collaboration
The Letter of Intent for International Axion Observatory was submitted to the CERN in August 2013. IAXO formally founded in July 2017 and received an advanced grant from the European Research Council in October 2018. The near-term goal of the collaboration is to build a precursor version of the experiment, called BabyIAXO, which will be located at DESY, Germany.

The IAXO Collaboration is formed by 21 institutes from 7 different countries.

Principle of operation
The IAXO experiment is based on the helioscope principle. Axions can be produced in stars (like the sun) via the Primakoff effect and other mechanisms. These axions would reach the telescope and would be converted into soft X-ray photons in the presence of a magnetic field. Then, these photons travel through a focusing X-ray optics, and are expected as an excess of signal in the detector when the magnet points to the Sun.



The limit to the axion-photon coupling is given by the FOM: $$\mathrm{g_{a\gamma}}^{4}  = B^2 L^2 A \times \epsilon_{d} b^{-1/2} \times \epsilon_{0} \alpha^{-1/2} \times \epsilon^{-1/2}_{t} t^{-1/2} $$ where the first factor is related to the magnet, with the magnetic field (B), the length of the magnet (L) and the area (A). The second part depends on the efficiency ($$\epsilon_d$$) and background (b) of the detector. The third is regarding the optics, more specifically the efficiency ($$\epsilon_0$$) and the area of the focused signal on the detector readout ($$\alpha$$). And the last term is related to the time (t) of operation.

IAXO: The International Axion Observatory


IAXO will be a next-generation enhanced helioscope, with a signal to noise ratio of five orders of magnitude higher compared to current-day detectors. The cross-sectional area of the magnet equipped with an X-ray focus optics is meant to increase this signal to background ratio. When the solar axions come in contact with the magnetic cross-section, they are converted into photons through the Primakoff effect. These photons would then be detected by X-ray detectors placed on the telescope. This implies that a larger magnetic cross-section will lead to a more intense signal. The magnet will be a purpose‐built large‐scale superconductor with a length of 20 meters and a field strength of up to 5.4 Tesla. The whole telescope will feature 8 bores (with 8 detection systems).

The helioscope will also be equipped with a mechanical system allowing it to follow the sun consistently throughout the day (about 50% of sun-tracking time), leading to enhanced exposure. The IAXO subsystems comprising magnets, optics, and detectors are planned to be fully optimized for solar axion detectors.

The FOM of IAXO would be over 300 times larger than in CAST, and the sensitivity of the axion-photon coupling measurement in would be 1–1.5 order of magnitude higher than that achieved by previous detectors.

BabyIAXO
BabyIAXO is a technological prototype of all the subsystems of the IAXO with 2 magnet bores (with 2 detection systems) in a magnet of 10 meters length. The prototype is a testing version and will serve as an intermediate step to explore further possible improvements to the final IAXO. BabyIAXO will be set up in Hamburg, Germany by the CERN and DESY collaboration. CERN will be responsible for giving in the design reports of prototype magnets and cryostat, and DESY will design and construct the movable platform along with the other infrastructure. The data taking by BabyIAXO is scheduled to start in 2028.

In addition to being a proof of concept for IAXO, BabyIAXO will have its own physics potential and a FOM around 100 times larger than CAST.

Sensitivity and physics potential
IAXO will primarily be hunting for solar axions, along with the potential to observe the quantum chromodynamics (QCD) axion in the mass range of 1 meV to 1 eV. It is also expected to be capable of discovering axion-like particles (known as the ALPs) coupled either with photons or electrons.

The QCD-axions and the ALPs are predicted to have quite similar properties, and hence IAXO, whose primary goal is to observe the solar axions and photon-coupled ALPs, will also be able to detect the QCD axions and ALPs from different unexplored astrophysical axion sources. It, therefore, has the potential to solve both the strong CP problem and the dark matter problem, which depends on the discovery of the axion particles.

IAXO is believed to be the most ambitious experiment among the current-day experiment set-ups to observe the hypothetical axions. It could also be later adapted to test models of hypothesized hidden photons or chameleons.

Sources accessible to IAXO
Any particle found by IAXO will be at the least a sub-dominant component of the dark matter. The observatory would be capable of observing from a wide range of sources given below.


 * 1) Quantum chromodynamics axions
 * 2) Dark matter axions
 * 3) Solar axions
 * 4) Axions from astrophysical cooling anomalies such as white dwarf cooling, neutron star cooling, globular clusters, and supergiant stars powered by helium.

Magnet
The central magnetic systems will have a large superconducting magnet, configured in a toroidal multibore manner, in order to generate a strong magnetic field over a larger volume. It will be a 10 meters long magnet consisting of two different coils made out of 35 km Rutherford cable. This configuration is calculated to generate a 2.5 Tesla magnetic field within a 70 cm diameter. The magnetic subsystem is inspired by the ATLAS experiment.

X-ray optics
Since BabyIAXO will have two bores in the magnet, two X-ray optics are required to operate in parallel. Both optics are Wolter telescopes (type I).

One of the two BabyIAXO optics will be based on a mature technology developed for NASA's NuStar X-ray satellite. The signal from the 0.7 m diameter bore will be focused to 0.2 $$\mathrm{cm^2}$$ area.

The second BabyIAXO optics will be one of the flight models of the XMM-Newton space mission that belongs to the ESA.

Detectors
IAXO and BabyIAXO will have multiple, diverse detectors working in parallel mounted to the different magnet bores.

The detectors for this experiment need to meet certain technical requirements. They need a high detection efficiency in the ROI (1 – 10 keV) where the Primakoff axion signal is expected. They also need very low background in ROI of under $$\mathrm{10^{-7} counts\,keV^{-1} cm ^{-2} s ^{-1} } $$ (less than 3 counts per year of data). To reach this background level, the detector relies on:


 * 1) The use of shielding both passive (to block environmental gammas) and active (to tag cosmic induce events).
 * 2) The intrinsic radiopurity of the construction materials.
 * 3) The advanced event discrimination strategies based on topological information, validated with simulations.

Detector technologies
Based upon the experience from CAST, the baseline detector technology will be a TPC with a Micromegas readout.

There are several other technologies under study: GridPix, Metallic Magnetic Calorimeters (MMC), Transition Edge Sensors (TES) and Silicon Drift Detectors (SDD).