Correspondence between HBT radii and the emission zone in non-central heavy ion collisions

TL;DR

This paper investigates how pion HBT radii measurements relate to the shape and orientation of the freeze-out distribution in non-central heavy ion collisions, using a realistic transport model to account for flow and non-Gaussian sources.

Contribution

It demonstrates that existing formulae can approximately connect HBT radii oscillations to the freezeout anisotropy in realistic collision scenarios.

Findings

HBT radii oscillations reflect freezeout anisotropy approximately.

Transport model shows formulae are valid beyond Gaussian sources.

Analysis provides insight into the space-time structure of the collision source.

Abstract

In non-central collisions between ultra-relativistic heavy ions, the freeze-out distribution is anisotropic, and its major longitudinal axis may be tilted away from the beam direction. The shape and orientation of this distribution are particularly interesting, as they provide a snapshot of the evolving source and reflect the space-time aspect of anisotropic flow. Experimentally, this information is extracted by measuring pion HBT radii as a function of angle with respect to the reaction plane. Existing formulae relating the oscillations of the radii and the freezeout anisotropy are in principle only valid for Gaussian sources with no collective flow. With a realistic transport model of the collision, which generates flow and non-Gaussian sources, we find that these formulae approximately reflect the anisotropy of the freezeout distribution.