Low Energy Antiproton Experiments -- A Review

Low energy antiprotons offer excellent opportunities to study properties of fundamental forces and symmetries in nature. Experiments with them can contribute substantially to deepen our fundamental knowledge in atomic, nuclear and particle physics. Searches for new interactions can be carried out by studying discrete symmetries. Known interactions can be tested precisely and fundamental constants can be extracted from accurate measurements on free antiprotons ($\bar{p}$'s) and bound two- and three-body systems such as antihydrogen ($\bar{{\rm H}}=\bar{p}e^-$), the antprotonic helium ion (He$^{++} \bar{p}$)$^+$ and the antiprotonic atomcule (He$^{++} \bar{p}e^-$) . The trapping of a single $\bar{p}$ in a Penning trap, the formation and precise studies of antiprotonic helium ions and atoms and recently the production of $\bar{{\rm H}}$ have been among the pioneering experiments. They have led already to precise values for $\bar{p}$ parameters, accurate tests of bound two- and three-body Quantum Electrodynamics (QED), tests of the CPT theorem and a better understanding of atom formation from their constituents. Future experiments promise more precise tests of the standard theory and have a robust potential to discover new physics.