International Axion Observatory

International Axion Observatory

IAXO Logo
Predecessor	CERN Axion Solar Telescope
Formation	July 2017 at DESY, Hamburg
Legal status	In construction
Purpose	Search for axions and other physics beyond the Standard Model
Headquarters	DESY, Hamburg, Germany
Fields	Astroparticle physics
Spokesperson	Igor G. Irastorza
Website	iaxo.desy.de

The International Axion Observatory (IAXO) is a next-generation axion helioscope for the search of solar axions and Axion-Like Particles (ALPs). It is the follow-up of the CERN Axion Solar Telescope (CAST), which operated from 2003 to 2022.^[1] IAXO will be set up by implementing the helioscope concept bringing it to a larger size and longer observation times.^[2][3][4]

The Letter of Intent for International Axion Observatory was submitted to the CERN in August 2013.^[5] IAXO formally founded in July 2017 and received an advanced grant from the European Research Council in October 2018.^[6] 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.^[1][7][8][9]

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

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 helioscope 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 potential of the experiment can be estimated by means of the figure of merit (FOM), which can be defined as ${\displaystyle FOM\sim B^{2}L^{2}A\times \epsilon _{d}b^{-1/2}\times \epsilon _{o}\alpha ^{-1/2}\times \epsilon _{t}^{1/2}t^{1/2}}$, where the first factor is related to the magnet and depends on the magnetic field (B), the length of the magnet (L) and the area of the bore (A). The second part depends on the efficiency (${\displaystyle \epsilon _{d}}$) and background (b) of the detector. The third is related to the optics, more specifically the efficiency (${\displaystyle \epsilon _{o}}$) and the area of the focused signal on the detector readout (${\displaystyle \alpha }$). The last term is related to the time (t) of operation. The objective is to maximise the value of the figure of merit in order to optimise the sensitivity of the experiment to axions.

IAXO will primarily be searching 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 ALPs.^[1] Therefore, IAXO will have the potential to solve both the strong CP problem and the dark matter problem.^[2][10][11].

It could also be later adapted to test models of hypothesized hidden photons or chameleons.^[2][3] Also, the magnet can be used as a haloscope to search for axion dark matter.

IAXO will have a sensitivity to the axion-photon coupling 1–1.5 order of magnitude higher than that achieved by previous detectors.^[1]

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][5]

1. Solar axions.
2. QCD axions.
3. Dark matter axions.
4. Axions from astrophysical hints such as white dwarf and neutron star anomalous cooling, globular clusters, and supergiant stars powered by helium.^[1]

IAXO will be a next-generation enhanced helioscope, with a signal to noise ratio five orders of magnitude higher compared to current-day detectors. The cross-sectional area of the magnet equipped with an X-ray focusing optics is meant to increase this signal to background ratio. When the solar axions interact with the magnetic field, some of them may convert into photons through the Primakoff effect. These photons would then be detected by the X-ray detectors of the helioscope.

The magnet will be a purpose‐built large‐scale superconductor with a length of 20 m and an average field strength of 2.5 Tesla. The whole helioscope will feature 8 bores of 60 cm diameter. Each of the bores will be equipped with a focusing X-ray optic and a low-background X-ray detector. The helioscope will also be equipped with a mechanical system allowing it to follow the sun consistently throughout half of the day. Tracking data will be taken during the day and background data will be taken during the night, which is the ideal split of data and background for properly estimating the event rate in each case and determining the axion signal.

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 m 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.^[12][13] 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.^[11][12][14]

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.

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.^[1][5]

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

One of the two BabyIAXO optics will be based on a mature technology developed for NASA's NuStar X-ray satellite.^[5] The signal from the 0.7 m diameter bore will be focused to 0.2 ${\displaystyle \mathrm {cm^{2}} }$ area.

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

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 ${\displaystyle \mathrm {10^{-7}counts\,keV^{-1}cm^{-2}s^{-1}} }$ (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.

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).

• CERN Axion Solar Telescope

• International Axion Observatory Website