Antihydrogen, the bound state of an antiproton and a positron, is the simplest anti-atom consisting purely of antimatter. Since ordinary hydrogen is one of the best measured physical systems, spectroscopic studies of antihydrogen promise a high sensitivity for tests of the matter-antimatter symmetry. This symmetry seems to be absent in the Cosmos where no antimatter is observed, in contrast to the Microcosm where the CPT theorem of the Standard Model of particle physics predicts a perfect symmetry between particles and antiparticles. Comparing the properties of hydrogen and antihydrogen with high precision can shed light onto these questions and give hints on physics beyond the Standard Model.
Several experiments at the Antiproton Decelerator of CERN are studying antihydrogen. An overview of status will be given with the focus on the ASACUSA collaboration, which pursues a measurement of the ground-state hyperfine splitting in a beam, and also investigates hydrogen with the same apparatus to study Lorentz invariance violations.