The QCD Critical Point and Related Observables

Abstract

The search for the critical point of QCD in heavy-ion collision experiments has sparked enormous interest with the completion of phase I of the RHIC beam energy scan. Here, I review the basics of the thermodynamics of the QCD phase transition and its implications for experimental multiplicity fluctuations in heavy-ion collisions. Several sources of noncritical fluctuations impact the observables and need to be understood in addition to the critical phenomena. Recent progress has been made in dynamical modeling of critical fluctuations, which ultimately is indispensable to understand potential signals of the QCD critical point in heavy-ion collision. Keywords: The QCD phase diagram and heavy-ion collisionsOne of the most fascinating opportunities of heavy-ion collisions is the investigation of the phases of strongly interacting matter at finite temperatures and densities. At ultrarelativistic beam energies the interaction of the initial nuclei excites the vacuum and high energies are deposited in the reaction zone, creating a very hot medium of vanishing net-baryon density. At lower beam energies the incoming baryon currents are stopped and the medium is less hot but more dense than at highest beam energies. In these extreme regimes it is expected that a new state of matter, the quark-gluon plasma of colored partonic degrees of freedom is created. The success of models like statistical hadronization [1] and of fluid dynamical simulations [2,3,4] in comparison to a variety of experimental data, indicates that the medium thermalizes locally and a connection to QCD thermodynamics is possible.Experimental efforts started at lower beam energies, from Bevalac and AGS to the CERN-SPS, mounting up to highest beam energies at RHIC and the LHC. Recent interest has focused on the possibility of scanning the QCD phase diagram by varying the beam energy in the lower energy region. The recent beam energy scan phase I has been completed at RHIC where ample data has been taken and analyzed [5,6,7,8]. Currently, the beam energy and system size scan with the NA61 experiment at the CERN-SPS has started and future programs include the beam energy scan phase II at RHIC and upcoming facilities like the CBM [9] experiment at FAIR, GSI Helmholtz Center and NICA [10] at JINR in Dubna. The main goal of these endeavors is the investigation of the QCD phase transition and the discovery of a conjectured critical point at finite net-baryon density.From lattice QCD calculations at zero baryo-chemical potential it is known that the phase change between a partonic and a hadronic medium is an analytic crossover [11] in the temperature range of T c =

The BEST framework for the search for the QCD critical point and the chiral magnetic effect

“…In parallel, the model of chiral fluid dynamics (CFD) was developed [91][92][93][94] (see Ref. [95] for an overview), and extended to a QCD-assisted transport approach [96] by using an effective potential beyond mean field and the sigma spectral function from functional renormalization group calculations [97]. In the CFD framework, the chiral condensate is identified as a dynamical variable while the slow modes relevant for the QCD critical point are related to conserved baryon densities (see Refs.…”

Hydrodynamic description of the baryon-charged quark-gluon plasma

“…In this section, we focus on cumulants of the baryon number, which are believed to be sensitive to the QCD critical point and thus could provide signatures of it in the near future when the data of the BES-II campaign are analyzed [32,38,129]. As a measurable and directly related observable, the net proton number cumulants can be obtained from protons and anti-protons multiplicities on an event-by-event basis.…”

The QCD critical point hunt: emergent new ideas and new dynamics

“…Meanwhile, the model of chiral fluid dynamics (CFD) has been developed over recent years [49][50][51] (see Ref. [52] for more references). In the CFD framework, the chiral condensate is identified as a dynamical variable.…”