System-size dependence of the pion freeze-out volume as a potential signature for the phase transition to a Quark Gluon Plasma

TL;DR

This study investigates how the pion freeze-out volume varies with system size and energy, revealing a minimum that may signal the transition to quark-gluon plasma, using transport models and correlation analysis.

Contribution

It identifies a non-monotonic behavior in pion freeze-out volume as a potential signature of the hadron-to-QGP transition at specific energies.

Findings

Minimum in pion freeze-out volume at 20-30A GeV energy.

The minimum is linked to the transition from hadronic to string matter.

Resonance decays and string fragmentation compete in particle production.

Abstract

Hanburry-Brown-Twiss (HBT) correlation functions and radii of negatively charged pions from C+C, Si+Si, Cu+Cu, and In+In at lower RHIC/SPS energies are calculated with the UrQMD transport model and the CRAB analyzing program. We find a minimum in the excitation function of the pion freeze-out volume at low transverse momenta and around $E_{lab}\sim 20-30A$GeV which can be related to the transition from hadronic to string matter (which might be interpreted as a pre-cursor of the QGP). The existence of the minimum is explained by the competition of two mechanisms of the particle production, resonance decays and string formation/fragmentation.