Correspondence between Hanbury-Brown–Twiss radii and the emission zone in noncentral heavy ion collisions

Abstract

In noncentral collisions between ultrarelativistic 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 Hanbury-Brown--Twiss (HBT) radii as a function of angle with respect to the reaction plane. The connection between measured radius oscillations and the underlying geometry is necessarily model-dependent; many existing formulas are strictly valid only 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 formulas approximately reflect the anisotropy of the freeze-out distribution.