Talks

Prasad Hegde: Probing U_A(1) Symmetry Restoration with Domain-Wall Fermions

The axial U_A(1) symmetry is broken in QCD at all values of the temperature. Nonetheless, its breaking is expected to be strongly reduced in the high temperature, chirally symmetric phase of QCD due to the suppression or gradual disappearance of topologically non-trivial gauge field configurations. We examine the interplay between chiral symmetry restoration, U_A(1) symmetry breaking and the appearance/disappearance of non-trivial topological gauge field configurations. We performed calculations in QCD with degenerate light up and down quark masses, corresponding to a pion mass of about 200 MeV, and a physical strange quark mass. We use domain-wall fermions which enjoy an almost exact chiral symmetry even for a≠0, a being the lattice spacing. This has the advantage that we can probe the underlying mechanism of U_A(1) violation, and can provide evidence for a connection between U_A(1) breaking and topology. We find strong evidence for a correlation between the appearance of non-trivial topological gauge configurations and large differences in the scalar (S) and pseudoscalar (PS) correlators which belong to the same meson multiplet and would be equal and opposite, if U_A(1) symmetry would not be broken. Our measurements are in agreement with the qualitative picture that U_A(1) is broken by non-trivial topological configurations whose density decreases as the temperature increases. Thus, the symmetry is only fully restored at T=∞.

Anirban Lahiri: Fluctuations and Correlations of Conserved Charges in the PNJL Model

We have computed fluctuations and correlations between conserved charges - baryon number (B), electric charge (Q) and strangeness (S). We have compared our leading order result with that of lattice QCD data and then predicted the behaviour of some more higher order fluctuations and correlation coefficients. We will discuss their implications on crossover and phase transitions of strongly interacting matter.

Sarbani Majumder: Strongly Interacting Matter Under Charge Neutrality and Beta equilibrium Conditions

We study the strongly intearcting matter under beta equilibrium and electrically neutral condition within 3 flavour PNJL model. Implication of these conditions on the QCD phase diagram is also investigated.

Bedangadas Mohanty: Possible evidence for thermalization at RHIC

QCD phase diagram is a type of graph which is used to show the equilibrium conditions between the thermodynamically distinct phases (partonic and hadronic). In order to study the phase diagram it is important to show experimentally that the system had reached thermodynamical equilibrium. A thermalized state is believed to be a maximum entropy state, to directly show this experimentally for a dynamic system as in heavy-ion collisions is difficult. One of the ways to demonstrate that the system has reached some degree of thermalization is by showing that the systems space-momentum distributions have reached equilibrium values. This can be done by comparing observables like direct photon and hadron momentum spectra, particle ratios, and products of moments of quantities related to conserved charges in QCD to model calculations which assume thermalization. Another way is to show that interactions among the constituents are large or have reached a saturation value. This can be studied by looking at correlations, such as in transverse momentum or those measured through elliptic flow. We will discuss some of the above observables in heavy-ion collisions at RHIC in the context of thermalization. Then provided few key measurements in the heavy flavour sector which are intended to be carried out at RHIC to establish thermlaization for light quark sector.

Basanta Nandi: ρ⁰ vector-meson elliptic flow (v₂) measurement in STAR experiment at RHIC

The study of elliptic flow (v₂) of the short-lived resonances provides a sensitive tool to probe the hot and dense medium produced in relativistic heavy ion collisions. It has been proposed that the measurement of v₂ of the resonances can distinguish whether the resonance was produced at hadronization via quark coalescence or later in the collision via hadron re-scattering. The ρ⁰ vector-meson is one among such resonances which has a very short life time with respect to the life time of the system formed in heavy-ion collisions. Therefore, the measurement of ρ⁰ v₂ can potentially provide information on the ρ⁰ production mechanism in relativistic heavy-ion collisions. In the intermediate p_T range (1.5 < p_T < 5 GeV/c), the elliptic flow parameter v₂, shows a deviation from the particle mass ordering for different hadron species. For identified hadrons, v₂ is found to follow a scaling with the number of constituent quarks n, which is expected from the quark coalescence model. ρ⁰ being a meson, its v₂ is expected to follow the n=2 in the universal curve of v₂(p_T/n) vs p_T/n. On the other hand, if ρ⁰ is produced from the π⁺π^- scattering during hadronization, it would follow the n=4 quark scaling (i.e. 2 for each pions). We will discuss the first time measurement of ρ⁰ elliptic flow in Cu+Cu and Au+Au collisions at &sqrt;s_NN = 200 GeV using the STAR Time Projection Chamber (TPC) and STAR Forward Time Projection Chamber (FTPC). The methods used in this measurement will be presented in the conference.

Helmut Satz: The QCD Phase Structure at High Baryon Density

We consider the QCD phase structure if color deconfinement and chiral symmetry restoration do not coincide in dense baryonic matter. This leads to a state of massive constituent quarks as intermediate phase between confined matter and the quark-gluon plasma.

Sayantan Sharma: A new method for computation of quark number susceptibilities in QCD

Computing higher order quark number susceptibilities(QNS) is important for the accurate de- termination of the critical end-point of QCD by Taylor series method. Moreover various diagonal and off-diagonal QNS help us to determine the properties of the quark-gluon plasma. By intro- ducing the chemical potential in the staggered fermion operator as a Lagrange multiplier associated with the point split number density term, we show that the computations of the QNS become faster. Furthermore, the QNS computed in this method are not prescription dependent as seen for the com- monly used methods. However, the second order susceptibility has a contribution which diverges in the limit of vanishing lattice spacing, a and corrections of different orders in a in the higher order QNS. We suggest a prescription to eliminate the divergence in second order susceptibility and the unwanted finite terms in the higher order QNS. We compute the various QNS on the lattice, for two flavour QCD with staggered fermions at NT = 6. Our method yields estimates of all the QNS consistent with the values computed using the standard method for the QGP phase, but with considerably less computational effort.

Sreekanth V: Thermal photons in QGP and non-ideal effects

We investigate the thermal photon production-rates using one dimensional boost-invariant second order relativistic hydrodynamics to find proper time evolution of the energy density and the temperature. The effect of bulk-viscosity and non-ideal equation of state are taken into account in a manner consistent with recent lattice QCD estimates. It is shown that the non-ideal gas equation of state i.e ε-3P≠0 behaviour of the expanding plasma, which is important near the phase-transition point, can significantly slow down the hydrodynamic expansion and thereby increase the photon production-rates. Inclusion of the bulk viscosity may also have similar effect on the hydrodynamic evolution. However the effect of bulk viscosity is shown to be significantly lower than the \textit{non-ideal} gas equation of state. We also analyze the interesting phenomenon of bulk viscosity induced cavitation making the hydrodynamical description invalid. We include the viscous corrections to the distribution functions while calculating the photon spectra. It is shown that ignoring the cavitation phenomenon can lead to erroneous estimation of the photon flux.

Brijesh Srivastava: Percolation and the Phase Transition

Heavy ion collisions are currently described in terms of color strings stretched between the nucleons of projectile and the target, which decay into new strings through q-qbar pairs production and subsequently hadronize to produce observed hadrons. Due to the confinement, the color of these strings is confined to small area in transverse space. With growing energy and/or atomic number of colliding nuclei, the number of strings grows and they start to overlap forming clusters, very much like disks in two dimensional percolation theory. At a certain critical density a macroscopic clusters appears marking the onset of the percolation phase transition. The STAR beam energy scan program at RHIC offers an excellent opportunity to study this.

Nu Xu: High-Energy Nuclear Collisions and QCD Phase Structure

One of the most exciting goals for the field of the high-energy nuclear collisions is to understand the phase structure of matter with partonic degrees of freedom and the transition from hadronic phase to partonic phase. The QCD phase structure dominates the evolution briefly during the early time of the Universe. In this talk, after reviewing basic concepts and recent progresses in the field, I will discuss the physics programs with Beam Energy Scan at RHIC.