Evidence for chiral symmetry restoration in heavy-ion collisions

P Moreau,A Palmese,W Cassing,E Seifert,T Steinert,E.L Bratkovskaya

doi:10.1016/j.nuclphysa.2017.05.030

Abstract

We study the effect of the chiral symmetry restoration (CSR) on heavy-ion collisions observables in the energy range sNN=3–20GeV within the Parton-Hadron-String Dynamics (PHSD) transport approach. The PHSD includes the deconfinement phase transition as well as essential aspects of CSR in the dense and hot hadronic medium, which are incorporated in the Schwinger mechanism for particle production. Our systematic studies show that chiral symmetry restoration plays a crucial role in the description of heavy-ion collisions at sNN=3–20GeV, realizing an increase of the hadronic particle production in the strangeness sector with respect to the non-strange one. Our results provide a microscopic explanation for the horn structure in the excitation function of the K+/π+ ratio: the CSR in the hadronic phase produces the steep increase of this particle ratio up to sNN≈7GeV, while the drop at higher energies is associated to the appearance of a deconfined partonic medium. Furthermore, the appearance/disappearance of the horn structure is investigated as a function of the system size. We additionally present an analysis of strangeness production in the (T,μB)-plane (as extracted from the PHSD for central Au+Au collisions) and discuss the perspectives to identify a possible critical point in the phase diagram.

Highlights

In this contribution we summarize the results from our study in Ref. [1] that investigates the strangeness enhancement in nucleus-nucleus collisions [2, 3] or the ’horn’ in the K+/π+ ratio [4, 5]
Moreau et al / Nuclear Physics A 00 (2021) 1–4 obtained a striking improvement on this issue when including chiral symmetry restoration (CSR) in the string decay for hadronic particle production [9]
Our studies are performed within the Parton-Hadron-String Dynamics (PHSD) transport approach that has been described in Refs. [10, 11]

Summary

Introduction

In this contribution we summarize the results from our study in Ref. [1] that investigates the strangeness enhancement in nucleus-nucleus collisions [2, 3] or the ’horn’ in the K+/π+ ratio [4, 5]. [1] that investigates the strangeness enhancement in nucleus-nucleus collisions [2, 3] or the ’horn’ in the K+/π+ ratio [4, 5]. Both phenomena have been addressed to a deconfinement transition. The actual experimental observation could not be described within conventional hadronic transport theory [6, 7, 8] and remained a major challenge for microscopic approaches. The Parton-Hadron-String Dynamics (PHSD), a transport approach describing heavy-ion collisions on the basis of partonic, hadronic and string degrees-of-freedom, P. Moreau et al / Nuclear Physics A 00 (2021) 1–4 obtained a striking improvement on this issue when including chiral symmetry restoration (CSR) in the string decay for hadronic particle production [9]

Chiral symmetry restoration in the string fragmentation process

The scalar quark condensate