Transverse radii from pion interferometry for an expanding fireball

Abstract

We apply pion-interferometry to a hydrodynamical model for particle production in high energy nuclear collisions. For an expanding sphere the HBT correlation function is calculated and effective transverse radii are extracted. The effects of collective flow, thermal gradients and different freeze-out surfaces are seen in the K T-dependence of the extracted radii. An ambiguity between flow and thermal gradient effects is found to render a model-independent extraction of the transverse flow velocity from HBT data impossible. In the SPS energy domain we always find smaller HBT radii R side if the system is allowed to develop hydrodynamic transverse flow than it if decouples before flow sets in. However, the transverse HBT radii are always larger than those of the cold projectile ions, in agreement with experimental findings. We explain this effect as a consequence of thermalization rather than collective expansion of the source.