RHIC Beam Energy Scan Research Articles

Identified hadron production from the RHIC beam energy scan

A current focus at the RHIC is the beam energy scan to study the QCD phase diagram—temperature (T) versus baryon chemical potential (μB). The STAR experiment has collected data for Au+Au collisions at 7.7, 11.5 and 39 GeV in the year 2010. We present midrapidity results on rapidity density, average transverse mass and particle ratios for identified hadrons from the STAR experiment. Collision dynamics are studied in the framework of chemical and kinetic freeze-out conditions.

STAR highlights

We report selected results from STAR collaboration at the RHIC, focusing on jet–hadron and jet-like correlations, quarkonium suppression and collectivity, di-electron spectrum in both p+p and Au+Au, and higher moments (standard deviation, skewness and kurtosis) of net-protons as well as azimuthal anisotropy from the RHIC Beam Energy Scan program.

Energy dependence of the freeze-out eccentricity from the azimuthal dependence of HBT at STAR

Non-central heavy-ion collisions create an out-of-plane-extended participant zone that expands toward a more round state as the system evolves. The recent RHIC beam energy scan at of 7.7, 11.5 and 39 GeV provides an opportunity to explore the energy dependence of the freeze-out eccentricity. The new low-energy data from STAR complement high statistics data sets at of 62.4 and 200 GeV. Hanbury Brown–Twiss (HBT) interferometry allows us to determine the size of pion-emitting source regions. The dependence of the HBT radius parameters on the azimuthal angle relative to the reaction plane has been extracted. These dependences can be related to the freeze-out eccentricity. The new results from STAR are consistent with a monotonically decreasing freeze-out eccentricity and constrain any minimum, suggested by previously available data, to lie in the range between 11.5 and 39 GeV. Of several models, UrQMD appears to best predict the STAR and AGS data.

Reaction Plane and Beam Energy Dependence Of The Balance Function at RHIC

The balance function, which measures the correlation between opposite sign charge pairs, is sensitive to the mechanisms of charge formation and the subsequent relative diffusion of the balancing charges. The study of the balance function can provide information about charge creation time as well as the subsequent collective behavior of particles. In this paper, we present a reaction-plane-dependent balance function study for Au+Au collisions at = 200 GeV and compare with results from recent three particle correlation measurements. We also report balance functions for relative pseudorapidity (Δη), relative rapidity (Δy), and relative azimuthal angle (Δϕ) from the recent RHIC beam energy scan data.

EVENT-BY-EVENT FLUCTUATIONS OF NET-BARYON DISTRIBUTION AND HIGHER ORDER CUMULANTS

We analyze fluctuations of net baryon distribution as well as its higher order cumulants, the 2rd order(c2), 4th order cumulants(c4) and other variant high order cumulants like Skewness, Kurtosis, R4,2 as a function of the collision centrality. At the same time, we study the results of transverse momentum and rapidity cut effects of all observables. We argue that Skewness and Kurtosis are promising observables in experiments considering to their independence of acceptance. Beam energy dependences of Skewness and Kurtosis have been studied, these works provide baseline predictions for the forthcoming RHIC Beam Energy Scan program.

RHIC beam energy scan program—experimental approach to the QCD phase diagram

A brief overview of the scientific program of the beam energy scan (BES) at RHIC, with emphasis on the search for the critical point (CP) of the QCD phase diagram, is presented. A more detailed discussion can be found in STAR Collaboration (Note no 493). The complementary experiments at other facilities are briefly discussed at the end of this paper.

Energy dependence of directed flow inAu+Aucollisions from a multiphase transport model

The directed flow of charged hadron and identified particles has been studied in the framework of a multi-phase transport (AMPT) model, for $^{197}$Au+$^{197}$Au collisions at $\sqrt{s_{NN}}=$200, 130, 62.4, 39, 17.2 and 9.2 GeV. The rapidity, centrality and energy dependence of directed flow for charged particles over a wide rapidity range are presented. AMPT model gives the correct $v_1(y)$ slope, as well as its trend as a function of energy, while it underestimates the magnitude. Within the AMPT model, the proton $v_1$ slope is found to change its sign when the energy increases to 130 GeV - a feature that is consistent with ``anti-flow''. Hadronic re-scattering is found having little effect on $v_1$ at top RHIC energies. These studies can help us to understand the collective dynamics at early times in relativistic heavy-ion collisions, and they can also be served as references for the RHIC Beam Energy Scan program.