During a first stage of the project, the work focused on consolidating the international collaboration, bringing the design of all parts of the BabyIAXO axion helioscope to the point of start of construction, as well as preparing the management structure and the adequate host environment at DESY-Hamburg. A first design of BabyIAXO was completed, enhancing its figure-of-merit according to the project’s goal, and showing the viability of its construction and operation as a fully-fledged axion helioscope with relevant physics case. After that, first construction actions have started, although with some delay with respect the original plan, especially regarding the large 10-m long superconducting magnet needed for the experiment. The latter is due primarily the difficulties to procure the special type of superconducting conductor needed for the magnet, not available in industry at the time. Only in a later stage of the project a realistic construction roadmap has been established, and a more detailed and improved design of the BabyIAXO magnet was successfully reviewed in 2024.
In other fronts, the project has satisfactorily achieved the required objectives. The collaboration has secured the endorsement of the European Space Agency (ESA) to use one existing 70 cm x-ray optics (XMM spare optics) in one of the magnet bores. An additional custom-made optic is being built within the IAXO collaboration for the second bore. Several prototypes of the low-background Micromegas x-ray detector for BabyIAXO have been built and tested as part of the project. One of them was commissioned underground at the Laboratorio Subterráneo de Canfranc (LSC), showing record background levels and reaching the BabyIAXO target. Another one, running above-ground at CAPA-Zaragoza, has defined the roadmap towards actively tagging cosmic-induced background events down to the required level. Yet another one (dubbed IAXO pathfinder), took axion-sensitive data in the CAST experiment at CERN, producing a new world-record upper limit to the axion-photon coupling in 2024. The project has also produced the needed software/analysis tooling needed for BabyIAXO (dubbed REST-for-Physics).
In addition, the physics case of the experiment has been substantially extended. A comprehensive "physics potential" review was published in 2019. Numerous studies have since then contributed to expand its physics impact. A particularly relevant case is the RADES concept, proposing "haloscope-like" setups to also search for dark matter axions with BabyIAXO. A conceptual study of a haloscope inside the BabyIAXO magnet was published in 2023. This initial side-topic of the project has grown in relevance to the point of constituting the seed of what later has become a successful ERC-SyG project (DarkQuantum) to start in 2024. This will boost and extend the physics reach of BabyIAXO and is considered a big success of the present project.
In summary, even if the BabyIAXO infrastructure is not yet completed, the project has successfully demonstrated its technical viability, has gathered the needed community momentum and support, and has placed the project well on-track under construction. As anticipated in the proposal, the grant has had a catalysing effect in attracting additional resources and cohering the efforts from the international collaboration. Most goals have been successfully accomplished, and BabyIAXO evolves to a fully-fledged axion helioscope experiment in itself (not just a prototyping stage of IAXO) with relevant physics potential.
