Laser spectroscopy of antiprotonic atoms and pionic atoms
Dr. Masaki Hori (Max-Planck-Institut für Quantenoptik, Garching)

Antiprotonic helium is a 3-body atom made of a helium nucleus, an electron, and an antiproton trapped in a Rydberg orbital.
The antiproton can be resonantly excited by irradiating the atom with high-power laser beams. We have recently used the Antiproton Decelerator facility of CERN to measure 13 transition frequencies of this atom to a precision of 2.5 to 15 parts per billion by laser spectroscopy. For this 2 billion atoms were cooled to a temperature of T=1.5-1.7 K by allowing them to collide with a buffer helium gas. By comparing the results with quantum electrodynamics (QED) calculations, the antiproton-to-electron mass ratio was determined as 1836.1526734(15). This result is in good agreement with the proton-to-electron value determined in Penning trap experiments carried out by the Mainz-GSI-Heidelberg groups, which constitutes a consistency test of CPT symmetry.
Pions are the lightest meson which are particles composed of two valance quarks. Within classical nuclear theory, they mediate the strong force that binds the nucleons together. Numerous atoms containing pions have been synthesized for many years, but no laser excitation has ever been achieved because of their very short (<1 ps) lifetimes. Such experiments should nevertheless allow us to determine the pion mass. Metastable pionic helium is a hypothetical Rydberg atom composed of a helium nucleus, an electron, and a negatively-charged pion. We are attempting to excite pionic transitions in this atom using an optical parametric generator (OPG) laser, in an experiment at the Ring Cyclotron facility of the Paul Scherrer Institute.