Collective flow has historically been an indicator that nuclear matter created in heavy ion collisions has undergone a phase change to a novel state where its constituent particles are deconfined. This phase, called a Quark-Gluon Plasma (QGP), has many characteristics that are signature of its creation. Chief among these is the collective behavior of the nuclear matter indicated by its anisotropic flow, as well as high pT particle suppression, baryon enhancement at mid-p T, and the enhancement of strange quark containing particles above binary scaling expectations. Recent results from the Large Hadron Collider (LHC) show evidence of collective flow in the simpler p+Pb system, implying that a QGP may be formed in smaller systems than previously thought. An elliptic flow measurement with identified particles in d+Au collisions could reveal more about the nuclear matter created in these simpler systems. The Pioneering High Energy Nuclear Ion Experiment, or PHENIX, Time of Flight detector used in conjunction with its Aerogel Cherenkov Counter can provide particle identification with good proton/kaon/pion separation for pT
We present the first measurements of identified hadron production, azimuthal anisotropy, and pion interferometry from Au+Au collisions below the nominal injection energy at the Relativistic Heavy-Ion Collider (RHIC) facility. The data were collected using the large acceptance STAR detector at (square root)s{sub NN} = 9.2 GeV from a test run of the collider in the year 2008. Midrapidity results on multiplicity density (dN/dy) in rapidity (y), average transverse momentum (p{sub T}), particle ratios, elliptic flow, and HBT radii are consistent with the corresponding results at similar (square root)s{sub NN} from fixed target experiments. Directed flow measurements are presented for both midrapidity and forward rapidity regions. Furthermore the collision centrality dependence of identified particle dN/dy, p{sub T}, and particle ratios are discussed. These results also demonstrate that the capabilities of the STAR detector, although optimized for (square root)s{sub NN} = 200 GeV, are suitable for the proposed QCD critical point search and exploration of the QCD phase diagram at RHIC.
This new volume, I/23, of the Landolt-Börnstein Data Collection series continues a tradition inaugurated by the late Editor-in-Chief, Professor Werner Martienssen, to provide in the style of an encyclopedia a summary of the results and ideas of Relativistic Heavy Ion Physics. Formerly, the Landolt-Börnstein series was mostly known as a compilation of numerical data and functional relations, but it was felt that the more comprehensive summary undertaken here should meet an urgent purpose. Volume I/23 reports on the present state of theoretical and experimental knowledge in the field of Relativistic Heavy Ion Physics. What is meant by this rather technical terminology is the study of strongly interacting matter, and its phases (in short QCD matter) by means of nucleus-nucleus collisions at relativistic energy. The past decade has seen a dramatic progress, and widening of scope in this field, which addresses one of the chief remaining open frontiers of Quantum Chromodynamics (QCD) and, in a wider sense, the "Standard Model of Elementary Interactions". The data resulting from the CERN SPS, BNL AGS and GSI SIS experiments, and in particular also from almost a decade of experiments carried out at the "Relativistic Heavy Ion Collider"(RHIC) at Brookhaven, have been fully analyzed, uncovering a wealth of information about both the confined and deconfined phases of QCD at high energy density.
Using the RQMD model, transverse momentum dependence of the anisotropic flow v2 for [pi], K, nucleon, [phi], and [lambda], are studied for Au + Au collisions at √s{sub NN} = 200 GeV. Both hydrodynamic hadron-mass hiragracy (hhmh) at low p{sub T} region and particle type dependence (baryon versus meson) at the intermediate p{sub T} region are reproduced with the model calculations although the model underpredicted the overall values of v2 by a factor of 2-3. As expected, when the rescatterings are turned off, all v2 becomes zero. The failure of the hadronic model in predicting the absolute values of hadron v2 clearly demonstrate the need of early dense partonic interaction in heavy-ion collisions at RHIC. At the intermediate p{sub T}, the hadron type dependence cold also be explained by the vacume hadronic cross sections within the frame of the model. The measurements of collective motion of hadrons from high-energy nuclear collisions can provide information on the dynamical equation of state information of the system [1, 2, 3]. Specifically, the strange and multi-strange hadron flow results have demonstrated the partonic collectivity [5] and the heavy-flavor flow will test the hypothesis of early thermalization in such collisions [4]. At RHIC, the measurements [6, 7] of elliptic flow v2 and nuclear modification factor r{sub AA} has lead to the conclusion that hadrons were formed via the coalescence/recombination of massive quarks [8, 9, 10]. This finding is directly related to the key issue in high-energy nuclear collisions such as deconfinement and chiral symmetry restoration. In addition, it also touched the important problem of hadronization process in high-energy collisions. Therefore a systematic study with different approaches becomes necessary. In this report, using a hadronic transport model UrQMD(v2.2)/RQMD(v2.4) [11, 12], we study the v2 of [pi], K, p, [phi], and [Lambda] from Au + Au collisions at 200 GeV. Properties of centrality dependent and freeze-out time dependent will be discussed. We try to answer some specific questions like how much the observed features can be reproduced by the hadronic model and why. In this approach, the vacumme cross sections are used for strong interactions. Unlike the treatment in most hydrodynamic calculations, the transition from strong interaction and free-steaming is determined by the local density and gradual. As we will discuss in the paper, the shortcoming of this method is lack of the partonic interactions which is important for the early dynamics in ultra-relativistic heavy ion collisions [13]. In order to take care of both partonic and hadronic interactions in high-energy nuclear collisions, a combination of hydrodynamic model for early stage (the perfect fluid stage) and hadronic transport model for later stage and freeze-out has been tried [14, 15].
Measurements of collective flow and two-particle correlations have proven to be effective tools for understanding the properties of the system produced in ultrarelativistic nucleus-nucleus collisions at the Relativistic Heavy Ion Collider (RHIC). Accurate modeling of the initial conditions of a heavy ion collision is crucial in the interpretation of these results. The anisotropic shape of the initial geometry of heavy ion collisions with finite impact parameter leads to an anisotropic particle production in the azimuthal direction through collective flow of the produced medium. In "head-on" collisions of Copper nuclei at ultrarelativistic energies, the magnitude of this "elliptic flow" has been observed to be significantly large. This is understood to be due to fluctuations in the initial geometry which leads to a significant anisotropy even for most central Cu+Cu collisions. This thesis presents a phenomenological study of the effect of initial geometry fluctuations on two-particle correlations and an experimental measurement of the magnitude of elliptic flow fluctuations which is predicted to be large if initial geometry fluctuations are present. Two-particle correlation measurements in Au+Au collisions at the top RHIC energies have shown that after correction for contributions from elliptic flow, strong azimuthal correlation signals are present at A0 = 0 and A0 ~ 120. These correlation structures may be understood in terms of event-by-event fluctuations which result in a triangular anisotropy in the initial collision geometry of heavy ion collisions, which in turn leads to a triangular anisotropy in particle production. It is observed that similar correlation structures are observed in A Multi-Phase Transport (AMPT) model and are, indeed, found to be driven by the triangular anisotropy in the initial collision geometry. Therefore "triangular flow" may be the appropriate description of these correlation structures in data. The measurement of elliptic flow fluctuations is complicated by the contributions of statistical fluctuations and other two-particle correlations (non-flow correlations) to the observed fluctuations in azimuthal particle anisotropy. New experimental techniques, which crucially rely on the uniquely large coverage of the PHOBOS detector at RHIC, are developed to quantify and correct for these contributions. Relative elliptic flow fluctuations of approximately 30-40% are observed in 6-45% most central Au+Au collisions at s NN= 200 GeV. These results are consistent with the predicted initial geometry fluctuations.
We have studied the dependence of azimuthal anisotropy v2 for inclusive and identified charged hadrons in Au+Au and Cu+Cu collisions on collision energy, species, and centrality. The values of v2 as a function of transverse momentum pT and centrality in Au+Au collisions at √sNN=200 and 62.4 GeV are the same within uncertainties. However, in Cu+Cu collisions we observe a decrease in v2 values as the collision energy is reduced from 200 to 62.4 GeV. The decrease is larger in the more peripheral collisions. By examining both Au+Au and Cu+Cu collisions we find that v2 depends both on eccentricity and the number of participants, Npart. We observe that v2 divided by eccentricity (?) monotonically increases with Npart and scales as N1/3part. Thus, the Cu+Cu data at 62.4 GeV falls below the other scaled v2 data. For identified hadrons, v2 divided by the number of constituent quarks nq is independent of hadron species as a function of transverse kinetic energy KET=mT–m between 0.1KEsubT/sub/nsubq/sub1 GeV. Combining all of the above scaling and normalizations, we observe a near-universal scaling, with the exception of the Cu+Cu data at 62.4 GeV, of vsub2/sub/(nsubq/sub∙????????Nsup1/3/supsubpart/sub) vs KEsubT/sub/n
In the last century, matter was confirmed to be made up from molecules which consist of two atoms or more. The atom itself consists of a nucleus made of protons and neutrons, and electrons "circling'' around the nucleus. The number of electrons or protons distinguish different elements. Later on, protons and neutrons were found not to be elementary particles but rather composite particles. The question turned then to be what are protons and neutrons made of and this is the focus of elementary particle physics. According to the standard model, protons and neutrons are made up of quarks and gluons. The theory that describes quarks and gluons is called quantum chromodynamics (QCD). According to this theory, quarks and gluons can not be detected freely; they appear only inside hadrons but are never observed freely (confinement). However, at high temperatures and/or densities a transition may happen where quarks and gluons do not exist in bound states (hadrons) anymore but rather exist freely (the asymptotic freedom). This phase of the nuclear matter is known as the quark-gluon plasma (QGP).To learn more about the QCD phase diagram, mainly the confinement and de-confinement transition, many different experiments have been performed from fixed target experiments to high-energy heavy-ion collisions in almost three decades. The discovery of QGP came from ultrarelativistic heavy-ion collision (URHIC) experiments. By ultrarelativistic heavy-ion collisions, we mean heavy ions like gold or lead that have been accelerated to speeds which are close to the speed of light (the ion momentum is much larger than its rest mass). Nowadays, ultrarelativistic heavy-ion collision experiments at the Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) are being used to create and study the quark-gluon plasma. From the early days after confirming the existence of the QGP, relativistic hydrodynamics has been used to describe the hadron spectra and collective flow seen in these experiments and has been quite successful. Since then, different approaches have been developed to model the physics of the QGP. The first approach used was ideal hydrodynamics where the QGP is assumed to behave like a perfect fluid with no viscosity. However, improvements in both the experimental and theoretical sides demonstrated the importance of including dissipative (viscous) effects in QGP modeling. The resulting relativistic viscous hydrodynamics models have been quite successful in describing the data. Despite this success, studies found that the QGP generated in URHICs is a highly momentum-space anisotropic plasma which means that viscous hydrodynamics will break down in some situations. To take this into account, anisotropic hydrodynamics (aHydro) was developed. In aHydro, one includes the momentum-space anisotropies in the distribution function at leading-order, whereas viscous hydrodynamics is expanded around the isotropic distribution function as the leading term and the viscous effects are included as correction terms. In this study, we present a new method for imposing a realistic equation of state in anisotropic hydrodynamics which is called quasiparticle anisotropic hydrodynamics (aHydroQP). In this method, we introduce a single finite-temperature quasiparticle mass which is fit to QCD lattice data. By taking moments of the Boltzmann equation assuming an anisotropic distribution function, we obtain a set of coupled partial differential equations which can be used to describe the 3+1d spacetime evolution of the QGP. Due to the numerical difficulties and the need to understand this new method more, instead of considering the 3+1d case immediately, we begin by studying two simpler cases. First, we specialize to the case of a 0+1d system undergoing boost-invariant Bjorken expansion and compare with the standard method of imposing the equation of state in anisotropic hydrodynamics (aHydro). We find practically no differences between the two methods results for the temperature evolution and the scaled energy density. When we compare the pressure anisotropy, we see only small differences, however, we find significant differences in the evolution of the bulk pressure correction. Second, we present the results in azimuthally-symmetric boost-invariant (1+1d) systems and compare the quasiparticle model with the standard aHydro model and second order viscous hydrodynamics. We compare the three methods' predictions for the primordial particle spectra, total number of charged particles, and average transverse momentum for various values of the shear viscosity to entropy density ratio. We show that they agree well for small shear viscosity to entropy density ratio, but show clear differences at large values of shear viscosity to entropy density ratio. Third, and most importantly, we present the phenomenological predictions of 3+1d quasiparticle anisotropic hydrodynamics compared with LHC 2.76 TeV Pb-Pb collisions. We present comparisons of charged-hadron multiplicity, identified-particle spectra, identified-particle average transverse momentum, charged-particle elliptic flow, identified-particle elliptic flow, elliptic flow as a function of pseudorapidity, and HBT radii. We find good agreement when compared with ALICE data. Looking to the future, we plan to include next-leading-order anisotropic hydrodynamics corrections by including the off-diagonal terms of the anisotropy tensor in quasiparticle anisotropic hydrodynamics. However, since this will be very hard and numerically intense, we consider first next-leading-order anisotropic hydrodynamics using the standard method for imposing the equation of state. To do so, we Taylor-expand assuming small off-diagonal terms to make the formalism easier and numerically tractable. Then, by taking moments of the Boltzmann equation, we find the dynamical equations needed to model the full 3+1d system. In this part of the work, we present only the theory setup and leave the numerical analysis for a future work.
