Positron lifetime distributions in fluids

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 favorable fluctuations. The fact that the experimental measurements yield a single, well-defined decay rate for each decay mode suggests 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 quantum Monte Carlo methods show that substantial fluctuations occur in the lp environment. In this paper, the distribution of fluctuations is described. It is shown that the apparent conflict is resolved if the time scale for fluctuations in the environment is much shorter than the duration of each measurement, and estimates for the different relaxation processes are provided.