Bulletin of the American Physical Society
APS April Meeting 2010
Volume 55, Number 1
Saturday–Tuesday, February 13–16, 2010; Washington, DC
Session X10: RHIC Heavy Ion Physics |
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Chair: Anne Sickles, Brookhaven National Laboratory Room: Maryland B |
Tuesday, February 16, 2010 10:45AM - 10:57AM |
X10.00001: Search for QCD critical point through fourth moments of net-proton distributions with the STAR experiment Xiaofeng Luo Fluctuations of conserved quantities like net-baryon number are considered to be signatures for the search for Quark Gluon Plasma formation and the QCD critical point. It has been proposed that higher moments of the net-proton distribution are more sensitive to the existence of a critical point compared to measures based on second moments [1]. In Lattice calculations, which assume the system is in thermal equilibrium, the fourth moments of event-by-event net baryon, net charge and net strangeness are related to respective susceptibilities. These susceptibilities become large at the critical temperature and are expected to increase in the presence of the QCD critical point. Recent theoretical calculations suggests that fluctuations in net- proton reflects the fluctuations in net-baryon. We present the energy and centrality dependence of higher moments of the net-proton distribution measured by the STAR experiment at RHIC. The measurement is carried out at mid-rapidity ($|y|< 0.5$) for Au+Au collisions at $\sqrt{s_{NN}}$ = 19.6, 62.4 and 200 GeV. The results are compared to calculations from UrQMD, AMPT, HIJING and Therminator models. These results will help to understand the expectations from scenarios where a QCD critical point is not incorporated in the models. \\[4pt] [1] M. A. Stephanov,\emph{ Phys. Rev. Lett.} \textbf{102}, 032301(2009). [Preview Abstract] |
Tuesday, February 16, 2010 10:57AM - 11:09AM |
X10.00002: Energy Dependence of the \textit{v}$_{2}$ scaling and the QCD Phase Boundary Nu Xu In high-energy nuclear collisions at RHIC ($s_{NN}$\textit{ = 60 -- 200 GeV}), the quark coalescence has been identified as the process for hadronization. As a result, one observes a scaling in elliptic flow parameter \textbf{\textit{v}}$_{2}$ [1,2] and hadron type dependence (within 2 $<$ p$_{T} \quad <$ 5 GeV/c region) in nuclear modification parameter \textbf{\textit{R}}$_{AA}$ [1]. In a given collision when the center of mass energy is not sufficiently high to create partonic matter, one would not expect the scaling in the final observed \textbf{\textit{v}}$_{2}$. Hence, the scaling provides us a sensitive tool in order to search for the possible phase boundary in the hot/dense matter dominated by either partonic or hadronic degrees of freedom. In this talk, we will report the results from analyzing the energy dependence of \textbf{\textit{v}}$_{2}$ for identified hadrons from Au+Au collisions for energy ranging from 200 GeV to 5 GeV. Data from transport models are used in our analysis. Without the partonic coalescence, the scaling is absent.\\[4pt] [1] J. Adams, \textit{et al.,} (STAR Collaboration), Nucl. Phys. \underline {\textbf{A757}}, 102(2005).\\[0pt] [2] B.I. Abelev, \textit{et al.,} (STAR Collaboration), Phys. Rev. Lett. \underline {\textbf{99}}, 112301(2007). [Preview Abstract] |
Tuesday, February 16, 2010 11:09AM - 11:21AM |
X10.00003: Charged hadron spectra in Cu+Cu collisions at 22.4 GeV with STAR at RHIC Orpheus Mall STAR has studied heavy ion collisions of Au+Au and Cu+Cu at different energies in order to probe the QGP phase boundary as a function of $T$ and $\mu_{B}$. We present a study of the Cu+Cu 22.4 GeV test run at the RHIC injection energy with $\sim1$ million good events. Particle spectra are presented for $\pi\pm$, $K\pm$, $p$ and $\bar{p}$ versus $m_{T}-m_{0}$ for $|y|<0.1$ at various collision centralities. We present the integrated yield $dN/dy$, $\langle p_{T} \rangle$ and particle ratios as a function of centrality. Kinetic freeze-out properties will be studied via blast-wave model fits to the spectra. Chemical freeze-out properties will be studied using a statistical model fit of the ratios to determine $T_{ch}$ and $\mu_{B}$. Results are compared with RHIC and world data for other energies and species. [Preview Abstract] |
Tuesday, February 16, 2010 11:21AM - 11:33AM |
X10.00004: Strangeness Production in 19.6 GeV Collisions at RHIC Kyle Englund-Krieger Particles carrying strange quarks carry information about the matter produced in high-energy nuclear collisions. Previous measurements of strange particle yields and spectra have proved useful as gauges of mid-rapidity baryon density, partonic collectivity, and strangeness enhancement and saturation. In the coming year, RHIC will begin a beam energy scan in search of evidence for a possible critical point in the phase diagram of nuclear matter. High precision measurements of strangeness production in lower energy collisions at RHIC and comparisons with high energy measurements will provide deeper understanding the evolution of nuclear matter with beam energy. We present mid-rapidity transverse momentum spectra for $\Lambda$, Anti-$\Lambda$, and $K^{0}_{s}$ short particles from $\sqrt{s_{NN}}=19.6$ GeV Au+Au collisions collected by the STAR experiment at RHIC. These results, from the second lowest beam energy used at RHIC so far, represent an initial step in the search for a critical point in the nuclear phase diagram and help fill-in the gaps in our understanding of strangeness production across a wide range of energy from AGS to RHIC. Our results will be compared with previous measurements from SPS, top RHIC energy, and with available model calculations. [Preview Abstract] |
Tuesday, February 16, 2010 11:33AM - 11:45AM |
X10.00005: Results from the lowest energy ($\sqrt{s_{NN}} =$ 9.2 GeV) collisions at RHIC Lokesh Kumar In order to search for the QCD critical point and explore the QCD phase Diagram, RHIC will soon begin a detailed Beam Energy Scan. The plan is to collide ions at various center-of-mass energies so as to explore the plane of temperature vs. baryon chemical potential. In preparation for the first major period of Energy Scan running, a short Au + Au test run at $\sqrt{s_{NN}} = 9.2$ GeV was conducted in the year 2008. The large-acceptance STAR detector collected about 3000 good events and results based on this data set will be presented. We will present results on the centrality dependence of identified particle (pion, kaon and proton) transverse momentum spectra, midrapidity multiplicity density ($dN/dy$), average transverse momentum ($\langle p_T \rangle$) and particle ratios. We will specifically discuss the energy dependence of the $K/\pi$ ratio, and compare to various available model calculations. The identified particle ratios and $p_T$ spectra will be used to extract chemical and kinetic freeze-out parameters, based on a thermal model fits to the data. The extracted freeze-out temperatures and baryonic chemical potential will also be compared with estimates from lower and higher beam energies. Finally, a brief overview of STAR's proposed plan for the Beam Energy Scan at RHIC will be presented. [Preview Abstract] |
Tuesday, February 16, 2010 11:45AM - 11:57AM |
X10.00006: Directed flow of identified particles for Au+Au collisions at 200 GeV Jiayun Chen Directed flow (v1) describes the dynamics from the pre-equilibrium stage of heavy ion collisions, and it has been suggested as a signal of a first order phase transition [1]. In particular, the v1 slope of protons at mid-rapidity is predicted to be very small due to anti-flow [2], it can even become negative if the anti-flow is strong enough. In the picture of baryon stopping with positive space-momentum correlation [3], the v1 slope of protons at mid-rapidity has the opposite sign of pions. In this talk, we will present STAR's measurements of v1 for pions, kaons (Ks), protons and anti-protons, for Au + Au collisions at 200 GeV. We found that the slope of proton v1(y) at midrapidity is extremely small. We will compare our result to model predictions, and we will examine our finding with the connection to both anti-flow and baryon stopping. \\[4pt] [1] H. Stocker, Nucl. Phys. A 750, 121 (2005) \\[0pt] [2] L. P. Csernai and D. Rohrich, Phys. Lett. B 458, 454 (1999); J. Brachmann, Phys. Rev. C 61, 024909 (2000). \\[0pt] [3] R. J. M. Snellings, H. Sorge, S. A. Voloshin, F. Q. Wang, and N. Xu, Phys. Rev. Lett. 84 (2000) 2803; H. Liu, S. Panitkin and N. Xu, Phys. Rev. C 59 (1998) 348. [Preview Abstract] |
Tuesday, February 16, 2010 11:57AM - 12:09PM |
X10.00007: Hadron Blind Detector implementation during PHENIX Run-10 Benji Lewis The Hadron Blind Detector (HBD), a windowless proximity focusing Cherenkov detector, in the PHENIX experiment at RHIC underwent restoration before the 2009-2010 physics run (Run-10). The impact of the HBD repairs on Run-10 will be addressed. In addition, a method of determining HBD electron identification efficieny in Au+Au collisions in Run-10 by merging HBD data from multiple events of p+p collisions taken during Run-9 will be discussed. [Preview Abstract] |
Tuesday, February 16, 2010 12:09PM - 12:21PM |
X10.00008: Baseline study for Chiral Symmetry Restoration using the Hadron Blind Detector in the PHENIX Experiment Sky Rolnick Measurement of the dielectron spectrum in p-p collisions at sqrt(s)=200GeV will be presented as a baseline study of Chiral Symmetry Restoration using the Hadron Blind Detector for the PHENIX experiment. Dielectrons offer us an ideal probe for studying ``medium'' modifications of vector mesons due to their color neutrality. Large combinatorial backgrounds, primarily from Dalitz decays and conversion pairs, limit the feasibility of such measurements. The Hadron Blind Detector has been specifically designed to rule out these backgrounds and offers a rejection factor of background of several orders of magnitude. In this study I will present various clustering algorithms and their behavior in high multiplicity environments such as central AuAu collisions as well as their expected rejection factors. This work should pave the road for very clean measurements of the dielectron continuum within the PHENIX experiment in the near future. [Preview Abstract] |
Tuesday, February 16, 2010 12:21PM - 12:33PM |
X10.00009: Dielectron mass spectra from $\sqrt{s_{NN}}$ = 200 GeV heavy ion collisions at PHENIX Sarah Campbell The dielectron mass spectrum consists of light vector meson decays, in addition to decays from other hadronic and photonic sources. Light vector mesons, modified by the medium via chiral symmetry restoration, and thermal radiation may provide additional signals at low masses above known hadronic sources. The PHENIX $\sqrt{s_{\rm NN}}$ = 200 GeV Au+Au analysis has measured a centrality dependent excess in the the low mass region (0.15 GeV/$c^{2}$ $<$ $m_{ee}$ $<$ 0.75 GeV/$c^{2}$) over the cocktail of known hadronic sources. The status of the PHENIX $\sqrt{s_{\rm NN}}$ = 200 GeV Cu+Cu analysis, in minimum bias and separated into centrality classes, will be shown, providing additional sensitivity in the study of this centrality dependent trend. [Preview Abstract] |
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