Exciting atoms containing mesons and antiprotons

Mesons are made of two quarks; they are highly unstable forms of matter, surviving for around a billionth of a second or less. Since 2014, researchers at the MPQ have used particle accelerators located at the Paul Scherrer Institute to synthesize artificial atoms that contain these mesons: here a pion and an electron orbit the nucleus of helium atoms. For the first time, the pion mass was determined by spectroscopy with infrared laser light with a very high precision. Likewise the mass of the antiproton relative to the mass of the electron was measured with high precision at CERN.