Antiprotonic Hydrogen: From Atomic Capture to Annihilation

Abstract

Antiprotons stopping in H2 form antiprotonic hydrogen, atoms of protons and antiprotons bound by their electromagnetic interaction. Antiprotonic hydrogen atoms are formed in high Rydberg states from which they deexcite by external Auger processes and by radiative transitions to the low-lying energy levels. The levels are shifted and broadened due to strong interactions between the two particles. The strong interaction shift and width of the 1S ground state and the width of the 2P state have been determined in recent experiments at the Low-Antiproton-Ring (LEAR) at CERN. The strong interaction effects can be understood quantitatively by including a meson exchange potential (known from nuclear forces) in the Hamiltonian. This meson exchange potential is known to require the existence of deuteron-like bound states of protons and antiprotons, often called baryonium. Baryonium resonances had beed searched for in numerous experiments but without any conclusive evidence for their existence. It is shown how a detailed understanding of the atomic cascade before annihilation has helped to find a new resonance which is likely such a baryonium resonance.