Does interferometry probe thermalization?

Abstract

We carry out a systematic study of interferometry radii in ultrarelativistic heavy-ion collisions within a two-dimensional transport model. We compute the transverse radii Ro and Rs as a function of pt for various values of the Knudsen number, which measures the degree of thermalization in the system. They converge to the hydrodynamical limit much more slowly (by a factor 3) than elliptic flow. This solves most of the HBT puzzle for central collisions: Ro/Rs is in the range 1.1-1.2 for realistic values of the Knudsen number, much closer to experimental data ( 1) than the value 1.5 from hydrodynamical calculations. The pt dependence of Ro and Rs, which is usually said to reflect collective flow, also has a very limited sensitivity to the degree of thermalization. We then study the azimuthal oscillations of Ro, Rs, and Ros for non central collisions. Their amplitudes depend little on the Knudsen number, and reflect the eccentricity of the overlap area between the two nuclei.