[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.

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

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

[1][5] IAXO[12] 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.

The helioscope will also be equipped with a mechanical system allowing it to follow the sun consistently throughout half of the day.

BabyIAXO[13] is an intermediate scale version of the IAXO experiment with axion discovery potential and a FOM around 100 times larger than CAST.

It will also serve as a technological prototype of all the subsystems of the helioscope as an first step to explore further improvements to the final IAXO experiment.

It will be a 10 m long magnet consisting of two different coils made out of 35 km Rutherford cable.

This configuration will generate a 2.5 Tesla magnetic field within the two 70 cm diameter bores.

One of the two BabyIAXO optics will be based on a mature technology developed for NASA's NuStar X-ray satellite.

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 and diverse detectors working in parallel, mounted to the different magnet bores.

Based upon the experience from CAST, the baseline detector technology will be a Time Projection Chamber (TPC) with a Micromegas readout.

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

They need a high detection efficiency in the ROI (1 – 10 keV) where the Primakoff axion signal is expected.