Bulletin of the American Physical Society
2012 Fall Meeting of the APS Division of Nuclear Physics
Volume 57, Number 9
Wednesday–Saturday, October 24–27, 2012; Newport Beach, California
Session HB: Mini-Symposium on Energy/Geometry Dependence of Relativistic Heavy Ion Collisions I |
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Chair: Steffan Bass, Duke University Room: Plaza II |
Friday, October 26, 2012 8:30AM - 9:06AM |
HB.00001: Energy/Geometry Dependence of Relativistic Heavy-Ion Collisions Invited Speaker: Giorgio Torrieri One of the main goals of heavy ion physics is to identify a change in intensive parameters (equilibrium or transport) of the system formed in heavy ion collisions which can be associated with the onset of deconfinement and appearance of partons as dynamical degrees of freedom. Because of the complexity of heavy ion collisions, such an unambiguous identification for a given energy and system size is at best extremely difficult. This goal, however, can be achieved if different energies, system sizes (both nucleus size and centrality), rapidities, momenta etc. are scanned across a wide range of parameters, and the results probed for scaling violations. In this talk, we will describe the current status and prospects of this search. We will show that both soft and hard observables seem to follow surprisingly simple scaling laws over a wide range of energies and geometries, simpler in fact than what can be naively expected. While explaining these scaling patterns might prove a challenge for the models widely used to describe these observables, the existence of the scaling itself suggests several promising experimental avenues to clarify its origin and search for deviations. [Preview Abstract] |
Friday, October 26, 2012 9:06AM - 9:18AM |
HB.00002: Hadron production and freeze-out dynamics in Au+Au at $\sqrt{s_{NN}}$ = 19.6 GeV Samantha Brovko The Beam Energy Scan program at RHIC was commissioned to search for the critical point and the turn-off of QGP signatures. The program completed collisions of Au+Au at energies ranging from 7.7 to 62.4 GeV per nucleon pair in the years 2010 and 2011. The addition of a full-coverage Time-of-Flight detector at STAR has extended the momentum range for clean particle identification. Mid-rapidity hadron spectra will be used to determine the freeze-out dynamics of the system. We will present particle spectra for $\pi$, K, p and $\bar{p}$ as a function of $m_{T}-m_{0}$ and use this to discuss particle ratios, in particular the source's Coulombic effect on soft pions, as well as chemical and kinetic freeze-out properties at $\sqrt{s_{NN}}$ = 19.6 GeV Au+Au. We will also compare results to $\sqrt{s_{NN}}$ = 7.7 GeV, 11.5 GeV, 19.6 GeV (from 2001), 27 GeV, 39 GeV Au+Au data from STAR, and $\sqrt{s_{NN}}$ = 17.3 GeV Pb+Pb data from the SPS heavy ion program. [Preview Abstract] |
Friday, October 26, 2012 9:18AM - 9:30AM |
HB.00003: The shapes of the multiplicity distributions in $\sqrt{s_{\rm NN}}$$=$7.7$-$200 GeV Au$+$Au Collisions at STAR Daniel McDonald A possible signature of the existence of a critical point in the phase diagram of nuclear matter is the non-monotonic behavior of the shapes of the multiplicity distributions of various groups of identified particles. These shapes are characterized by the variance and higher statistical moments, which may reflect the critical fluctuations that may diverge at beam energies near the critical point. The STAR experiment has measured Au$+$Au collisions at a wide range of beam energies, $\sqrt{s_{\rm NN}}$$=$7.7$-$200 GeV, and is well suited for numerous measurements because of its wide, uniform acceptance and the extended particle identification from a newly-installed Time-of-Flight (TOF) system. The measurements of the shapes of the multiplicity distributions of net protons and net charge, as well as total protons and total pions - via the intensive normalized cumulants of Ref. [1] and the moments products S$\sigma$ and K$\sigma^2$- will be described. \\[4pt] [1] C. Athanasiou {\it et al.}, Phys. Rev. D 82, 074008 (2010). M. Stephanov, Phys. Rev. Lett. 107, 052301 (2011). [Preview Abstract] |
Friday, October 26, 2012 9:30AM - 9:42AM |
HB.00004: Light (anti)nucleus production in $\sqrt{\rm s_{\rm NN}}$$=$7.7-200 GeV Au$+$Au collisions in the STAR Experiment W.J. Llope In the dense and high-temperature systems formed in relativistic heavy-ion collisions, final-state composites - light nuclei and antinuclei - are formed close to the freeze-out hypersurface. Their spectra, compared to those of the constituent (anti)nucleons, can be described by picturing the formation process as the coalescence of a number of nucleons that are close to each other in phase space. This makes the composite spectra sensitive to the distribution of the constituent nucleons in phase space. It also implies a sensitivity of the spectra to the local densities and flow velocities of the source. In the coalescence picture, specific ratios of these spectra provide information on the baryon densities and homogeneity volumes. The STAR experiment has collected data from Au$+$Au collisions at seven beam energies, $\sqrt{\rm s_{\rm NN}}$, ranging from 7.7 to 200 GeV. The particle identification is performed for transverse momenta from $\sim$0.3 to \raisebox{-0.6ex}{$\stackrel{>}{\sim}$}3 GeV/c using a combination of the ionization energy loss in the Time Projection Chamber and the time of flight. The spectra for (anti)protons, (anti)deuterons, and (anti)tritons at mid-rapidity, and the source information inferred from these spectra, will be presented and compared to models. [Preview Abstract] |
Friday, October 26, 2012 9:42AM - 9:54AM |
HB.00005: Energy Dependence of the High Moments from Transport Model Simulations Nu Xu, Xiaofeng Luo, Bedangadas Mohanty One of the most exciting goals for the field of high-energy nuclear collisions is to understand the phase structure of matter with partonic degrees of freedom especially the transition from hadronic to partonic matter, the quark-gluon plasma (QGP). In high-energy nuclear collisions at RHIC, the new form of matter, strongly interacting quark-gluon plasma (sQGP) has been formed. The question now is what is the structure of the QCD phase diagram in the region where the baryonic chemical potential is large. In this talk, we will report a systematic study of high-moments for net-charge, net-baryon and net-proton as a function of collision energies. These studies were performed with the AMPT and UrQMD models and the results will be compared with available experimental data. The main goal for this study is to prepare the reference where no dynamical effects, caused by either phase transition or QCD critical point, are expected. This is a necessary study for understanding the experimental results from the Beam Energy Scan program at RHIC. In addition, the study will also help us to understand the experimental effects such as charge conservations, acceptance and so on. [Preview Abstract] |
Friday, October 26, 2012 9:54AM - 10:06AM |
HB.00006: Transverse Energy in Forward/Backward Directions from RHIC Au+Au Collisions at Several Beam Energies Brett Fadem In 2010, RHIC produced Au+Au collisions at $\sqrt{s_{NN}}=200, 62.4, 39$, and $7.7 $~GeV. Progress in measuring transverse energy in the range $3.1<|\eta|<3.8$ using the PHENIX Muon Piston Calorimeter will be reported. Transverse energy has been used to estimate energy density in ultra-relativistic heavy ion collisions and to discriminate between competing models of hadronic interactions. At forward rapidities the net baryon densities are much higher than those at mid-rapidity, so one can probe these models at high baryon chemical potential. Furthermore, fluctuations in transverse energy might signal the presence of a critical point in the phase diagram of nuclear matter. [Preview Abstract] |
Friday, October 26, 2012 10:06AM - 10:18AM |
HB.00007: Fixed Target Results from STAR using Gold on Beampipe (Al) Events Christopher Flores, Daniel Cebra, Samantha Brovko, Brooke Haag, Jim Draper The RHIC Beam Energy Scan was proposed to search for the possible critical point and to study the nature of the phase transition between hadronic and partonic matter. However, several dynamical model simulations (UrQMD, PHSD, QGSM, GiBUU, 3-fluid) suggest that the partonic phase is entered for center-of-mass collision energies as low as 4-5 GeV [1]. Collisions between beam halo nuclei and the aluminum beam pipe allow STAR to study fixed-target Au+Al collisions. The injection and sub-injection energy gold beams (kinetic energies of 8.8, 4.8 and 2.9 AGeV) produce Au+Al collisions at center-of-mass energies of 4.5, 3.5, and 3.0 GeV. Particle ratios will be presented and compared to earlier published results from the AGS. Fixed target acceptances and efficiencies for tracking in the TPC and particle identification in the Time of Flight system will be shown.\\[4pt] [1] I. C. Arsene et al., Phys. Rev. C 75, 034902 (2007) [Preview Abstract] |
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