Session LJ: Mini-Symposium: Search for the Critical Point II

Longitudinal De-correlation of Anisotropic Flow in Au+Au Collisions at 27 GeV from STAR

Studies of longitudinal de-correlation of anisotropic flow provide unique constraints on the initial conditions and dynamical evolution of the quark gluon-plasma in heavy-ion collisions. The newly installed Event Plane Detector (EPD) in both forward and backward directions provides a unique opportunity to measure the flow de-correlation at STAR/RHIC. In this study, the factorization ratio for flow harmonics, $r_{n}(\eta^{a},\eta^{b})(n=2,3),$ is obtained over a wide $\eta$ range for 27 GeV Au+Au collisions as functions of centrality and transverse momentum. Comparing to results from LHC and 200 GeV Au+Au collision, a clear energy dependence is observed indicating a stronger longitudinal de-correlation at lower collision energies. The results may provide new insights into the three-dimensional modeling of the evolution of relativistic heavy-ion collisions and the shear viscosity of the QGP, especially their collision energy dependence.   

Determining Collision Event Centrality Using the Event Plane Detector at the STAR Experiment

Heavy ion physicists have been lately concerned with investigating the possibility of a critical point and a first order phase transition in the phase diagram for Quantum Chromodynamics (QCD). Most observables aimed at the determination of these phase properties rely on centrality determination, which is the degree of overlap for the colliding particles. The STAR experiment currently uses the Time Projection Chamber (TPC) to determine centrality; however, the TPC spans the same rapidity acceptance as many of the observables. This talk will focus on the potential to use the STAR Event Plane Detector (EPD), which is located at forward rapidity of $2.1<|\eta|<5.1$, for this purpose in order to prevent autocorrelations in analysis. We will show how well centrality determinations from these two detectors agree for Au+Au collisions at $\sqrt{s_{NN}}=27$ GeV.   

QCD critical point, universality, and small quark mass

The universality of the QCD equation of state near the critical point is expressed by mapping QCD pressure onto the Gibbs free energy in the Ising model. The mapping parameters are, in general, not universal, i.e., determined by the unknown details of the microscopic physics, rather than by symmetries and universal long-distance dynamics. We point out that in the limit of small quark masses, when the critical point is close to the tricritical point, the mapping parameters show certain {\em universal} dependence on the quark mass and discuss possible phenomenological consequences of these findings.   

Beam-energy dependence of spatial and temporal characteristics of shape-selected events in Au+Au collisions at STAR

The correlations measured from the Hanbury Brown and Twiss effect (HBT) allows access to the spatial and temporal characteristics of the systems produced in relativistic heavy-ion collisions. This presentation contains new measurements of the two-pion HBT radii, $\mathrm{R_{out}}$, $\mathrm{R_{side}}$ and $\mathrm{R_{long}}$ which have been made for shape-engineered events by the STAR experiment. Shape selection was accomplished via cuts on the distributions of the second-order flow vector $Q_2$. 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). The implications of these results for expansion dynamics of the collision systems will be discussed.   

Magnetic field in expanding quark-gluon plasma

Intense electromagnetic fields are created in the quark-gluon plasma by the external ultra-relativistic valence charges. The time-evolution and the strength of this field are strongly affected by the electrical conductivity of the plasma. Yet, it has recently been observed that the effect of the magnetic field on the plasma flow is small. We compute the effect of plasma flow on magnetic field and demonstrate that it is less than 10$\backslash ${\%}. These observations indicate that the plasma hydrodynamics and the dynamics of electromagnetic field decouple. Thus, it is a very good approximation, on the one hand, to study QGP in the background electromagnetic field generated by external sources and, on the other hand, to investigate the dynamics of magnetic field in the background plasma. We also argue that the wake induced by the magnetic field in plasma is negligible.   

A search for the magnetic field in the QGP by STAR

Lambda polarization $\bar{P}_{\Lambda/\bar{\Lambda}}$ was measured by the STAR collaboration, confirming the existence of extremely large vorticities within the Quark Gluon Plasma (QGP). Additionally suggested is an enhanced $\bar{P}_{\bar{\Lambda}}$ relative to $\bar{P}_{\Lambda}$ across all beam energies; however, the statistics are too limited to make a significant measurement. No such splitting is observed in the high-statistics $\sqrt{s_{\mathrm{NN}}} = 200$ GeV data set, but this splitting is expected increase at lower beam energies. Such a splitting in polarization would be consistent with the effects of hyperon magnetic-moment coupling with the magnetic field sustained in the QGP; it would have far-reaching consequences important to magnetic-field-dependent observables such as the chiral magnetic effect and would set the scale on the conductivity of the QGP. Recently, STAR has taken a high-statistics data set at $\sqrt{s_{\mathrm{NN}}} =$ 27 GeV which is considered suitable to study the splitting between $\bar{P}_{\Lambda}$ and $\bar{P}_{\bar{\Lambda}}$ since it includes the recently installed Event-Plane Detector (EPD), leading to a significantly increased event-plane resolution. We will present the measurement of this splitting and discuss its implications.   

System Size, Energy, and Centrality Dependence of the $\eta$ to $\pi^0$ Ratio

Data on the ratio of $\eta$ to $\pi^0$ with respect to transverse momentum $p_T$ from different experiments, different collisions systems ($p$+$p$, $p$+A, as well as A+A), and different center of mass energies $\sqrt{s_{NN}}$ are collected and compared to each other. We find that the ratio is surprisingly similar in all systems. We characterize and quantify the universality of the ratio and determine an empirical function for $\frac{\eta}{\pi^0}(p_T)$ including its systematic uncertainties. With this function we can derive the invariant yield for the $\eta$ meson based on $\pi^0$ measurements. Our procedure holds the promise to be more precise than the method of scaling with transverse mass ($m_T$), which is currently used in the PHENIX experiment. The new approach may reduce the systematic uncertainty on ongoing low $p_T$ direct photon measurements by PHENIX. In this talk we will present our method and the results.   

Effect of the GUP on the Entropy Density, Speed of Sound, and Bulk to Shear Viscosity Ratio of an Ideal QGP

One of the candidates to reconcile quantum mechanics with general relativity is the generalization of the Heisenberg Uncertainty Principle to incorporate gravitational effects. As a result, the Generalized Uncertainty Principle (GUP) "deforms" the commutation relation given by the Heisenberg Uncertainty Principle via a GUP parameter $\alpha$ . We present a calculation of the entropy density, speed of sound, and the resulting impact on the bulk viscosity to shear viscosity ratio of an ideal quark gluon plasma when the effects of the GUP are taken into consideration. When the GUP parameter tends to zero, we obtain the value of the speed of sound for an ideal gas of massless particles i.e. $c_s^2 =1/3$ and the expected result that the bulk viscosity vanishes. In addition, in the high temperature limit, the speed of sound tends to $c_s^2=1/4$ . The consequence this has on the bulk viscosity is that in the high temperature limit, the ratio of the bulk to shear viscosity tends to $\zeta/\eta=5/48$. Our results suggest that the GUP introduces a scale into the system breaking the a priori conformal invariance of a system of massless noninteracting particles. We sketch an attempt to find GUP modifications to the KSS bound.