POSITRONIUM IN XENON - THE PATH-INTEGRAL APPROACH

The ability of a single positronium atom to form a semimacroscopic bub ble in helium is a remarkable manifestation of quantum mechanics. Expe rimental evidence for bubble formation is provided by a dramatic decre ase in the decay rate of the triplet state when the ambient conditions favor its formation. The phenomenon is observed near the critical poi nt of helium and is well explained by a mean-field theory in which the positronium atom occupies the ground state of a local potential well induced by its influence on the average local density. Because of the active role played by the positronium atom in producing this localized state, the process is referred to as self-trapping. Similar experimen ts on other noble gases also show evidence for self-trapping near the liquid-vapor critical point, but the transition from extended to local ized behavior is gradual. Mean-field theories, which ignore fluctuatio ns, are not successful at these higher temperatures, suggesting that s tatistical fluctuations strongly influence the distribution over state s of the light atom. This paper presents a theoretical investigation o f the localization of positronium in a dense noble gas which employs t he path integral to represent the translational degrees of freedom of the light atom. It demonstrates that a theoretical model which properl y accounts for fluctuations is able to predict the main features of th e experimental measurements.