Antiprotonic Helium — An Exotic Hydrogenic Atom

Abstract

The antiprotonic helium,$\overline {\text{p}} {\text{e}}{\text{He}}^{\text{2}} + \left( { = \overline {\text{p}} {\text{He}}^{\text{ + }} } \right)$ , is a peculiar metastable atom, interfacing between matter and antimatter. A series of metastable states are composed of the He nucleus, one electron in the ground 1s configuration and one antiproton orbiting with large quantum numbers (n, l), where $n \sim l \sim \sqrt {{{M_{\overline p }^ * } \mathord{\left/ {\vphantom {{M_{\overline p }^ * } {m_e }}} \right. \kern-\nulldelimiterspace} {m_e }}} \sim 38$ . They possess a dual character as an exotic atom and an exotic diatomic molecule, and is often called antiprotonic helium atom-molecule, or for short, atomcule. From the chemical physics point of view the $\overline p He^ +$ may be regarded as an exotic neutral hydrogen atom with a composite “pseudo proton” with various effective charges, binding energies and spatial distributions. Since its discovery in 1991 at KEK comprehensive experimental studies have been carried out at CERN. In particular, the laser resonance spectroscopy of $\overline p He^ +$ has yielded the following results. 1) Precise determination of the transition energies to the precision of ppm. When compared with advanced theoretical predictions of the binding energies of this Coulomb 3-body system including the relativistic effects and QED corrections, the mass and charge of p have been determined with ppm precision. 2) Hyperfine structure of $\overline p He^ +$ due to the coupling of the electron spin with the large orbital magnetic moment of $\overline p$ has been revealed experimentally. 3) The dependence of the lifetimes of the individual (n, l) states of $\overline p He^ +$ on the He medium density, foreign atoms and molecules has been studied with the laser tagging method.