Session D16: Correlations and Fluctuations in Heavy-ion Collisions

Live

Correlations and fluctuations provide critical insight into the formation and evolution of the Quark Gluon Plasma (QGP), the initial state of collisions used to create the QGP, hadronization mechanisms, and the nature of the QCD phase transition. I will discuss the latest correlations and fluctuations results, and discuss what the implications are for strongly coupled matter created in heavy-ion collisions.   

On Demand

This talk describes the measurements of flow harmonics $v_2$--$v_6$ in Xe+Xe collisions at 5.44 TeV performed using the ATLAS detector at the LHC. Measurements of the centrality, multiplicity and $p_T$ dependence of the $v_n$ obtained using two-particle correlations and the scalar product technique will be presented. The measurements are also performed using a template-fit procedure, which was developed to remove non-flow correlations in small collision systems. This non-flow removal is shown to have a significant influence on the measured $v_n$ at high $p_T$, especially in peripheral events. Comparisons of the measured $v_n$ with measurements in Pb+Pb collisions and p+Pb collisions at 5.02 TeV are also presented. The $v_n$ values in Xe+Xe collisions are observed to be larger than those in Pb+Pb collisions for $n=2$, 3 and 4 in the most central events. However, with decreasing centrality or increasing harmonic order $n$, the $v_n$ values in Xe+Xe collisions become smaller than those in Pb+Pb collisions. The $v_n$ in Xe+Xe and Pb+Pb collisions are also compared as a function of the mean number of participating nucleons and the measured charged-particle multiplicity in the detector. Comparisons of the $v_n$ measurements with theoretical calculations will also be shown.   

Psuedothermalization of the quark-gluon plasma

We demonstrate that, within high-temperature pure gauge quantum chromodynamics, there exists a forward attractor for a large set of moments of the one-particle distribution function. Our results are obtained within the high-temperature effective kinetic theory approach which includes both elastic ($2 \leftrightarrow 2$) and inelastic ($2 \leftrightarrow 1$) contributions to the collisional kernel. We present results obtained using two different initial conditions corresponding to momentum-anisotropic thermal and over-occupied gaussian one-particle distribution functions. We compare the results obtained for the scaled moments of the one-particle distribution function with the attractor for kinetic theory in relaxation time approximation and two different versions of viscous hydrodynamics obtained using relaxation times that are momentum independent and momentum dependent. Our results indicate that the pseudothermal attractor which emerges in effective kinetic theory is different from both the relaxation time approximation attractor and both versions of viscous hydrodynamics.   

Not Participating

Elliptic flow is one of the most important observables in the relativistic heavy-ion collisions, which can allow us to access the collective properties of the expanding system. In this presentation, we will present elliptic flow of identified particles ($\pi^{\pm}$, $K^{\pm}$, $p$($\bar{p}$), $K_{S}^{0}$, $\Lambda$($\bar{\Lambda}$), $\phi$, $\Xi^{-}$($\bar{\Xi}^{+}$), $\Omega^{-}$($\bar{\Omega}^{+}$)) at midrapidity ($|$$\eta$$|$$<$1) as a function of transverse momentum in Au+Au collisions at $\sqrt{s_{NN}}$ = 27 and 54.4 GeV in the STAR experiment. High precision test of the number of constituent quark scaling of $v_{2}$ and the $v_{2}$ difference between particles and antiparticles will be shown. The $\phi$-meson and multistrange hadrons have small hadronic cross sections and freeze-out early from the medium, therefore can be used to study the energy dependence of partonic and hadronic interactions. Furthermore, the mass ordering of $v_{2}$ is expected to be violated between proton and $\phi$-meson in the low $p_{T}$ range ($p_{T}$ $<$ 1.5 GeV/c) due to their different sensitivity to hadronic phase. These results provide us an opportunity to study the hadronic contributions on $v_{2}$ measurements as a function of collision energy   

Not Participating

Experimental evidences at RHIC and LHC have demonstrated the formation of Quark-Gluon Plasma (QGP) in ultra-relativistic heavy-ion collisions at vanishing baryon chemical potential ($\mu_{B} \approx 0$ Mev) while the phase transition from the hadronic matter to QGP is suggested to be a crossover from state-of-the-art Lattice QCD calculations. It has been conjectured that there is a first-order phase transition and a critical point at finite $\mu_{B}$ region in the QCD phase diagram. In the search of possible QCD critical point at the QCD phase diagram, higher-order cumulants of conserved quantities (B, Q, S) as sensitive observables to locate its position. In this talk, we will present measurements of higher-order cumulants (up to the 4th order) of net-proton multiplicity distributions in Au+Au collisions at $\sqrt{s_{NN}}$=7.7, 11.5, 14.5, 19.6, 27, 39, 54.4, 62.4   

Not Participating

One of the fundamental goals of heavy-ion collision experiment is to map out the temperature and baryon chemical potential parameters of the Quantum Chromodynamics (QCD) phase diagram at which chemical freeze-out occurs – a point on the phase diagram when the chemical composition of the system is fixed. The cumulants of conserved quantities (net-charge, net-baryon, and net-strangeness) are directly related to the quark number susceptibilities calculated with lattice QCD. In a fluctuation analysis, the proxies for net-charge, net-baryon, and net-strangeness are net-pion, net-proton, and net-kaon respectively. The lambda particle consists of a strange quark and it is as well a baryon. Thus, the measurement of the cumulants of the net-lambda distributions can provide insight to the fluctuations of net-strangeness and net-baryon number. Moreover, by comparing the freeze-out parameters of the net-lambdas to that of the net-protons, inference can be made about what precipitates chemical freeze-out. That is, if chemical freeze-out is guided by the flavor of the quarks involved.   

Not Participating

A relativistic density functional parameterization of the QCD equation of state is used in a hadronic transport simulation of nuclear matter. The behavior of the system is analyzed in a number of scenarios, including initialization within the spinodal region of the QCD phase transition. An analysis of the dynamic evolution of observables expected to carry an experimental signal for the existence of the QCD critical point is performed.