Session DC: Beam Energy Scan and Critical Point

Charged Particle Distributions in Au+Au Collisions at $\sqrt{s_{NN}}=3.0$ GeV at STAR

The RHIC Beam Energy Scan phase I (BES-I) program provided a detailed study of nuclear matter over a wide range of energies. Below $\sqrt{s_{NN}}<19.6$ GeV, interesting results were shown in hadron azimuthal anisotropies, particle ratios, and net-proton higher moments which motivate the Beam Energy Scan phase II (BES-II). Compared to BES-I, BES-II has had improvements including increased statistics by a factor of 10 to 20 for each energy, improved acceptance from upgrades to the STAR experiment, and an extension of the energy reach from $\sqrt{s_{NN}}=7.7$ GeV to $\sqrt{s_{NN}}=3.0$ GeV with the STAR fixed-target program. This talk will present results from the lowest fixed target energy to be studied in BES-II including transverse mass spectra, rapidity density distributions, particle ratios, and centrality dependence for charged hadrons. These results are analyzed with a chemical equilibrium model to determine the chemical temperature and potential at freeze-out. At low energy, produced particles are sensitive to a Coulomb potential from a net positive source at low momentum which modifies the transverse mass spectra. The $\pi^{+}/\pi^{-}$ ratio and extracted Coulomb potential will also be presented. These new data are compared to published results from experiments at the AGS.   

beam-energy and collision-system dependence of flow correlations and fluctuations in heavy-ion collisions

Investigations of flow correlations and fluctuations in heavy-ion collisions can give in-depth insights into the expansion dynamics of these collisions. They can also provide new constraints for initial-state models to allow robust extraction of the specific shear viscosity $\eta/s$. The recent STAR differential measurements of the flow correlations (symmetric cumulants) and the flow-momentum correlations, $\rho(v^{2}_{n},\langle p_{T} \rangle)$~[1], will be presented for several collisions-systems at different beam energies. The results show characteristic system- and beam-energy-dependent trends which are compared with similar LHC measurements~[2,3] as well as calculations from several viscous hydrodynamic models. The comparisons between data and theoretical calculations show that the measurements can be used to pin down the respective influence of initial-state fluctuations, system-size, shape ($\varepsilon$), and $\eta/s(T)$. The implications of the constraining power of these measurements will be discussed.\\ ~\\ [1]~Piotr Bozek, Transverse momentum-flow correlations in relativistic heavy-ion collisions, Phys. Rev. C {\bf 93}, 044908 (2016).\\ [2]~ATLAS Collaboration, Measurement of flow harmonics correlations with mean transverse momentum in lead-lead and   

Critical behavior in mean-field hadronic transport

We develop a relativistic density functional parameterization of the QCD equation of state and use it in a hadronic transport simulation. We investigate the behavior of nuclear matter in a number of scenarios, including initialization within the spinodal region and in the vicinity of a critical point of the QCD phase transition. In particular, we analyze the time evolution of experimentally accessible observables sensitive to the existence of the QCD critical point.   

STAR Measurement of $\Lambda$-p Correlation in Search for the Chiral Vortical Effect in Au+Au Collisions at $\sqrt{s_{\rm NN}}$ = 27 GeV'', Brian Chan, UCLA, for the STAR Collaboration

Quantum Chromodynamics (QCD) allows for the formation of parity-odd domains inside the Quark-Gluon Plasma (QGP). The proposed Chiral Magnetic Effect (CME) is supposed to lead to charge separation along the direction of magnetic field created by spectator protons in the collision. Analogous to CME, theoretical calculations of the Chiral Vortical Effect (CVE) predict a difference in baryon-baryon versus baryon-antibaryon correlations along the direction of the vorticity. We will present the current status of measurements of $\Lambda$-proton azimuthal correlations in Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 27~GeV collected in 2018 with STAR detector to investigate the CVE predictions. Specifically, the $\gamma_{112}$ = $\langle$cos($\phi_{1}$ + $\phi_{2}$ - 2$\Psi_{\rm RP}$)$\rangle$ correlator is used to detect the signal of CVE, while $\gamma_{132}$ = $\langle$cos($\phi_{1}$ - 3$\phi_{2}$ + 2$\Psi_{\rm RP}$)$\rangle$ is used to model the background. We will also show the corresponding $\gamma$ correlator normalized by the expected flow background, which facilitates comparison with various models and across different colliding systems. The physics implications of our measurements will be discussed.   

Using machine learning to understand the properties of the QCD critical point

One of the main goals of the second phase of the Beam Energy Scan program at RHIC is to search for the QCD critical point. In order to study the thermodynamic effects of the presence of a critical point, we constructed a family of equations of state using a model that couples Lattice QCD results to a parameterized critical point from the 3D Ising model universality class. The mapping of the Ising critical point onto the QCD phase diagram gives rise to free parameters that control its position and size/shape of the critical region. We use machine learning to identify choices of free parameters that result in inconsistent thermodynamics. This approach can rule out pathological parameter sets at a low computational cost. Large scale implementation of our procedure can eliminate possible locations of the QCD critical point and guide experimental searches at RHIC.   

Explorations of the nuclear matter phase diagram via azimuthal anisotropy scaling functions

Azimuthal anisotropy scaling functions give unprecedented insights into the expansion dynamics of the fireballs created in ion-ion collisions at RHIC and the LHC. They also provide unmatched constraints for charting the phase structure of the QCD phase diagram, as well as the thermodynamic and transport properties of the respective QCD phases. I will first discuss the rudiments of Azimuthal Anisotropy Scaling Functions (AASF). Then, I will show that ALL of the existing anisotropy data at RHIC and the LHC, spanning different systems [large and small], beam energies, collision centralities, transverse momentum, identified particles, etc., lead to such scaling functions and their associated scaling coefficients. These scaling coefficients provide new and unique constraints for the detailed characterization of both the phase structure of the QCD phase diagram and the transport properties of its respective phases.   

Beam-energy dependence of spatial and temporal characteristics of shape-selected events in Au+Au collisions at the Relativistic Heavy Ion Collider(RHIC)

Newly measured two-pion Hanbury Brown and Twiss (HBT) radii have been made for shape-engineered events using data from the STAR experiment. These correlations leverage the HBT effect to allow investigation into how shape-engineering manifests itself within the spatial and temporal characteristics of the systems produced in relativistic heavy-ion collisions. Measureable changes in the HBT radii $\mathrm{R_{out}}$, $\mathrm{R_{side}}$ and $\mathrm{R_{long}}$ due to shape-engineering would be indicative of a change in the expansions dynamics of the system--ultimately helping to link initial state shape with final state measurements. Shape selection was accomplished via cuts on the distributions of the second-order flow vector $Q_2$ \footnote{J. Schukraft, A. Timmins, and S. A. Voloshin, Phys.Lett. B719, 394 (2013)}. Selected events, characterized with larger magnitudes of $Q_{2}$, indicate a systematic decrease for $R_{long}$ and $R_{out}$ with little, if any, change for $R_{side}$. Results obtained as a function of collision centrality and average pair transverse momentum ($k_T$) will be presented for the full range of the Au+Au beam energy scan ($\sqrt{s_{NN}} = 7.7 - 200$~GeV).   

Transverse energy analysis from identified particle transverse momentum spectra.

In high energy heavy ion collisions, the energy densities can often exceed the theoretical limit for the formulation of Quark gluon plasma (QGP). Measurements of the transverse energy production in heavy ion collisions provide valuable information about the global properties of these collisions and the initial energy state of the QGP. Traditionally, measurement of the transverse energy is obtained via calorimeter. Here we will present measurements using an alternative method, based on identified particle transverse momentum spectra, published by the STAR collaboration. For this analysis we use transverse momentum spectra corresponding to proton, pion, kaon and lambda, ranging from 7.7 GeV to 200 GeV. The assumptions of this analysis will be discussed, as well as comparisons to calorimeter transverse energy measurements published by the PHENIX collaboration.   

Critical Exponents from a Holographic Critical Point.

By using the holographic gauge/gravity correspondence, we construct a family of five-dimensional black holes to map the thermodynamics of strongly interacting matter. The black holes solutions, which are constrained to mimic the equation of state from lattice QCD at vanishing chemical potential, display a critical end point with a first order phase transition line. We calculate the critical exponents for the holographic critical point and compare them with other critical points from the literature.   

Signature of the QCD Critical Point from a Holographic Approach

We employ the equation of state with critical behavior obtained from the holographic model studied in [1] to compute higher-order susceptibilities of baryon number. We relate our susceptibilities to the moments of the distribution of net-protons measured in heavy-ion collision and discuss possible signatures of the critical point. In particular, we study the non-monotonic behavior of the kurtosis as the freeze-out trajectories move closer to the critical point. \begin{thebibliography}{99} \bibitem{c1} Phys.Rev.D 96 (2017) 9, 096026. \end{thebibliography}