Antiprotonic helium

Abstract

Antiprotonic helium, a neutral three-body system p ̄ e − He 2+ (= p ̄ He +) , produced when antiprotons ( p ̄ ) are captured in various phases of helium, comprises a series of long-lived ( ∼ μs lifetime) states in which the p̄ orbits the helium nucleus in a localized trajectory with large principal and orbital quantum numbers ( n, l). Typically, n∼ M ∗/m e ∼38 and l∼ n−1, where M ∗ is the reduced mass of the p̄–He system, whereas the electron occupies the ground 1s orbital. Because the p̄ moves slowly compared to the electron, this system possesses a dual character as both an exotic atom and a peculiar diatomic molecule, and is often called an antiprotonic helium atom–molecule, or atomcule for short. Its unique character of being metastable among p̄-containing atomic systems has permitted it to be studied with high-resolution laser spectroscopic techniques, yielding rich information on its formation, structure and decay. We first discuss the discovery at KEK Japan of the unexpectedly long survival time of p̄'s in liquid helium and the consequent systematic studies at the CERN LEAR facility of delayed annihilation time spectra of p̄'s stopped in various phases of helium. Theoretical models are introduced to explain the longevity and to predict the characteristic properties of antiprotonic helium. Laser spectroscopic studies have advanced and clarified the structure of the atomcule. These provide high-precision data on the transition energies which can be compared with advanced theories on the three-body Coulomb system. The latter now include QED effects to ppm precision, while the hyperfine structure has also been investigated experimentally and theoretically. Chemical physics aspects of antiprotonic helium are also described, including such environment-dependent effects as the dependence of its transition frequencies and level lifetimes on the phase and density of the surrounding helium, and the quenching of state populations by surrounding foreign atoms and molecules at impurity-level concentrations. The future scope of experiments on antiprotonic helium is discussed.