Does interferometry probe thermalization?

TL;DR

This study investigates how interferometry radii in ultrarelativistic heavy-ion collisions relate to thermalization, showing that certain radii are closer to experimental data than hydrodynamical models suggest, with limited sensitivity to thermalization levels.

Contribution

It provides a systematic analysis of interferometry radii dependence on thermalization, resolving the HBT puzzle and clarifying the limited impact of thermalization on certain observables.

Findings

R_o/R_s ratio aligns better with experimental data at realistic Knudsen numbers.

Transverse radii show slow convergence to hydrodynamical limit, explaining the HBT puzzle.

Azimuthal oscillations depend mainly on geometric eccentricity, not 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 R_o and R_s as a function of p_t 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: R_o/R_s is in the range 1.1-1.2 for realistic values of the Knudsen number, much closer to experimental data ($\simeq 1$) than the value 1.5 from hydrodynamical calculations. The p_t dependence of R_o and R_s, 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 R_o, R_s, and R_{os} for non central collisions. Their amplitudes depend little on the Knudsen number, and reflect the eccentricity of the overlap area between the two nuclei.