Based on our understanding of the universe, we, who are made of matter, should not exist. This is due to the fact that the subatomic world is largely indifferent between matter and antimatter, predicting that they would be created in almost equal amounts in a big-bang scenario, and subsequently annihilate to form pure energy. This conundrum is called the baryon asymmetry problem. It is one of the major and most pressing unsolved questions in science today. One way of addressing this issue is to look for deviations between precision measurements and Standard Model predictions. However, a major limitation arises in the stage of comparison with theory due to nuclear-structure effects. Conducting measurements with exotic atoms overcomes this limitation, either by comparing measured properties between atoms and anti-atoms directly with no input from theory, or by measuring systems composed of particles with no internal structure. QUARTET—Quantum transitions in exotic atoms, will pursue both directions through two complementary experimental campaigns:
1. Muonium spectroscopy using the most intense low-energy muon beam at PSI.
2. Antihydrogen spectroscopy using the most intense low-energy proton beam at the ELENA beamline at CERN.
QUARTET will focus on transitions in the microwave region of the electromagnetic spectrum, namely the classical Lamb Shift, which for Antihydrogen has an added value: If no difference between matter and its counterpart is observed, this measurement will provide the first determination of the antiproton charge radius.
