Status of laser spectroscopy of antiprotonic and pionic helium atoms

The Atomic Spectroscopy and Collisions Using Slow Antprotons (ASACUSA) collaboration at CERN's Antiproton Decelerator is carrying out laser spectroscopy experiments to measure the atomic transition frequencies of antiprotonic helium. Techniques such as sub-Doppler two-photon laser spectroscopy and buffer-gas cooling of the atoms to cryogenic temperature T=1.5--1.7 K are employed to measure the frequencies to a precision of 2.5 parts in $10^9$. By comparing the results with three-body quantum electrodynamics calculations, the antiproton-to-electron mass ratio was determined as 1836.1526734(15). The data has also been used to constrain exotic fifth forces that may exist at the 1 Angstrom length scale. Experiments were carried out to further improve the experimental precision in the last 3 years. Future experiments employing the high-quality antiproton beam provided by the Extra Low Energy Antiproton Ring (ELENA) facility now being commissioned at CERN will also be described. Pionic helium is a heretofore hypothetical long-lived atom composed of a helium nucleus, an electron, and a negatively-charged pion occupying a Rydberg state. Laser spectroscopy of this object was attempted in an experiment carried out using the high-intensity pion beam of the 590 MeV ring cyclotron facility of the Paul Scherrer Institute (PSI). Data analysis is ongoing.