Localisation of ortho-positronium in argon

Positronium lifetime measurements in argon are reported for the temperature range from 120K to room temperature and at pressures up to 220 bar. The results indicate the formation of a distinct bubble in the liquid phase and a more diffuse bubble in the gas phase. While at higher densities the experimental results can be rather well reproduced in terms of the density functional theory, at lower densities, below the theoretical bubble formation threshold, spontaneous density fluctuations in the fluid seem to explain the observed behaviour. Depending on the model for the free positronium energy in a homogeneous fluid, the Ps-Ar scattering length has a value between 0.5 and 0.8 AA.