Testing CPT invariance with antiprotonic atoms

The structure of matter is related to symmetries on every level of study. The CPT symmetry is one of the most important laws of the field theory: it states the invariance of physical properties when one simultaneously changes the signs of the charge and of the spatial and time coordinates of particles. Although in general opinion CPT symmetry is not violated in Nature, there are theoretical attempts to develop CPT-violating models. The antiproton decelerator at CERN has been built to test CPT invariance. Its three experiments compare the properties of particles and antiparticles by studying antihydrogen, the positron-antiproton bound system and the antiprotonic helium atom.