Probing the QCD phase structure using event-by-event fluctuations

TL;DR

This paper reviews how event-by-event fluctuations in heavy-ion collisions can reveal the QCD phase transition, critical point, and critical phenomena, combining experimental measurements and theoretical predictions.

Contribution

It discusses methods to extract thermodynamic response functions from fluctuations and proposes new approaches to map local temperature fluctuations.

Findings

Non-monotonic behavior in higher-order cumulants predicted by lattice QCD.

Experimental fluctuation measurements vary with collision centrality and energy.

Proposed mapping of temperature fluctuations in eta-phi plane to study local energy density.

Abstract

Heavy-ion collisions at relativistic energies probe matter at extreme conditions of temperatures and energy densities. The study of event-by-event fluctuations of experimental observables is crucial to probe the QCD phase transition, locate the critical point, and learn about the associated critical phenomena. At the critical point, all thermodynamic quantities behave anomalously. Fluctuation measurements provide access to thermodynamic response functions. We discuss the methods for obtaining the isothermal compressibility using particle multiplicity fluctuation, and specific heat using fluctuations in mean transverse momentum, temperature, and energy. Lattice QCD calculations have predicted non-monotonic behavior in the higher-order cumulants of conserved quantities at the critical point. Fluctuations in the multiplicity of charged to neutral particles have been measured to understand the formation of domains of disoriented chiral condensates. We review the recent fluctuation results as a function of collision centrality and energy from experiments at SPS, RHIC, and LHC. In addition, we propose to map the temperature fluctuations in eta-phi plane to probe local fluctuations of temperature and energy density.