The most precise search for a mysterious particle that could explain dark matter

Scientists have established the most restrictive limit to date in the search for axions, currently only theoretical particles that could be the key to solving two of the greatest mysteries of modern physics.

The work is the work of scientists from the Center for Astroparticles and High Energy Physics (CAPA) of the University of Zaragoza in Spain, in international collaboration with CERN (the European Laboratory for Particle Physics).

The results of the new study, published in the academic journal Physical Review Letters, represent the culmination of two decades of work in the international CAST (CERN Axion Solar Telescope) collaboration, demonstrating that, if axions exist, their interaction with light must be even weaker than expected. “This result not only improves our previous results, but also exceeds the limits derived from very competitive astrophysical arguments, such as those derived from the observation of stars in globular clusters,” explains Igor G. Irastorza, project leader at the University of Zaragoza. . “It is especially significant because, unlike astrophysical observations, our result is based on direct measurements made under controlled laboratory conditions.”

Axions are particles postulated in the late 1970s that could simultaneously explain the origin of dark matter and solve one of the deepest puzzles in particle physics, the “strong CP problem.” Axions are also ideal candidates to explain dark matter, that mysterious form of matter that constitutes approximately 85% of all matter in the universe and that, today, we can only detect due to its gravitational effects.

The CAST experiment makes use of a prototype magnet from the LHC particle accelerator converted into a very special telescope, capable of pointing at the solar core and converting axions into detectable X-rays. The CAPA group has managed to double the detection efficiency of these axions in the last phase of the CAST experiment. “During the development of my thesis, we have managed to significantly improve the sensitivity of the experiment,” says Cristina Margalejo, CAPA predoctoral researcher specialized in ultra-low background X-ray detectors. “This has allowed us to establish these very restrictive limits in the search for axions. The key has been the development of new analysis techniques and the use of new X-ray optics, which, together with a new gas mixture for our xenon-based detector, has allowed us to reduce the background noise where we expect it. the sign.”

Cristina Margalejo in the CAST experiment at CERN, where the most precise results to date have been achieved in the search for solar axions. (Photo: CAPA / University of Zaragoza / CERN)

The new limit established for axion-photon coupling is 5.8×10⁻¹¹ GeV⁻¹, which represents a substantial improvement over previous measurements. “The results can be visualized in what we call an exclusion graph, which is like a map where we discard the regions where the axion cannot live,” explains Jaime Ruz, an expert in X-ray optics with extensive experience in CAST. “With CAST we have managed to explore axion masses with a sensitivity that no other experiment had achieved before, approaching a very promising region where the theory predicts the main axion models, although we could also find other axionic-type particles outside this region” .

All the accumulated technical knowledge will be fundamental for IAXO (International Axion Observatory), the next major project in this field that is being built at the German Electron Synchrotron (DESY). The IAXO observatory, led by Igor G. Irastorza from the University of Zaragoza, will represent a qualitative leap in the search for axions, with a sensitivity much higher than that of CAST. The technical advances developed by the CAPA team, which leads the design and construction of the IAXO detection systems, will be crucial to the success of this new scientific infrastructure. (Source: CAPA / University of Zaragoza)