Path integral studies of positrons and positronium in fluids

Abstract
Experimental studies of positron and positronium decay in dense fluids suggest that these particles are capable of forming self-trapped States in some fluids in a broad region of temperature and density surrounding the liquid-vapor critical point. A central question in understanding the phenomena is whether the light particle (lp) actively creates a static potential well in the fluid in which it localizes, or randomly visits statistically favorable fluctuations. The fact that the experimental measurements yield a single, well defined, decay rate for each decay mode suggest that the environment of the lp is static. Earlier mean field theories could not shed much light on this question. However, two recent applications of Path Integral Monte Carlo show that substantial fluctuations occur in the lp environment. In this report, the two applications of path integration to lifetime studies are reviewed. The quantitative distribution of fluctuations in the decay rate for each particle (positron and positronium atom) is presented. It is shown that the apparent conflict between theory and experiment is resolved for positronium (but not positron) decay when the time scale for the relaxation of fluctuations in the environment is taken into account. Finally, recent results concerning the ability of the analytic reference interaction site model (RISM) polaron theory to reproduce the exact PIMC calculations are described.