Two breakthroughs of the early 20th century marked the beginning of modern physics: the formulations of quantum physics and special relativity. Combining the two concepts lead Dirac in 1928 to his famous equation, which predicted the existence of antimatter. Matter-antimatter pairs can be created from excess energy in a process called pair production. If all matter around us stems from pair production in the early universe it remains a mystery where all the antimatter has gone, since matter and antimatter should have been produced equally, but the universe is clearly not awash with antimatter.
An explanation could be found if the symmetric properties of matter-antimatter pairs are not as perfect as predicted by the Standard Model, our current best model of the universe. An excellent system to shed light on this question is antihydrogen, the simplest atomic system consisting purely of antimatter, being the bound state of an antiproton and a positron (the antiparticle of the electron). By comparing measured properties of antihydrogen to hydrogen, which is possibly the best measured composite structure in physics, we can identify a matter-antimatter asymmetry if the measured properties are different.
The Marietta Blau Institute of the Austrian Academy of Sciences is a member of the international ASACUSA collaboration (with partners from Japan, Italy, the UK, and Switzerland). We are part of the ASACUSA-Cusp experiment at CERN where the Antiproton Decelerator (AD) facility is the only source of ‘slow’ antiprotons in the world. Using these antiprotons and our own positron source, we produce an antihydrogen beam which we intend to use to perform microwave spectroscopy of the ground-state hyperfine-splitting. This quantity is expected to yield some of the most precise tests of matter-antimatter symmetry. The formation of the required beam of antihydrogen is an extremely challenging task, with an ‘intense’ antihydrogen beam only being achieved in 2024, after over 20 years of work.
