1,721,094 research outputs found
Radiative Transport and Heavy Ion Collisions
Full text linkThe good agreement of relativistic, dissipative hydrodynamics with heavy ion experiments at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider seems to indicate that the medium produced in the collision is near local equilibrium. However, the mechanism of the observed rapid equilibration is still an unsolved problem. While elastic 2 → 2 perturbative QCD rates do not quickly thermalize or produce large elliptic flow, it has been claimed that perturbative gg ↔ ggg rates can equilibrate rapidly. To investigate rapid equilibration via gg ↔ ggg, the partonic stage of a heavy ion collision is modeled using covariant kinetic theory. The key achievement of this thesis work is the implementation of leading order perturbative QCD matrix elements for gg ↔ scattering in a stochastic Boltzmann transport solver algorithm, in order to facilitate studies that include both elastic gg → gg and radiative gg ↔ ggg interactions. The role of the invariant matrix element and screening technique in equilibration and the generation of elliptic flow are then investigated by calculating the total cross section, equilibrium collision rates, and elliptic flow in Au + Au collisions at RHIC. This work has found that collision rates and elliptic flow results strongly depend on the choice of gg → ggg matrix element and screening technique
Jet energy loss in quark-gluon plasma
No full textThis dissertation discusses my investigations of energy loss of high-momentum quarks and gluons in the quark-gluon plasma (QGP). Two different jet energy loss mechanisms, elastic (collisional) energy loss and radiative energy loss, are studied extensively. Frame-independent treatments of Gyulassy-Levai-Vitev (GLV) radiative energy loss for light partons, Djordjevic-Gyulassy-Levai-Vitev (DGLV) radiative energy loss for heavy quarks, and Thoma-Gyulassy (TG) elastic energy loss, as well as of a simple dE/dL radiative energy loss, are formulated and applied to energy loss calculations for both light parton and heavy quark jets in heavy ion collisions at both Relativistic Heavy-ion Collider (RHIC) and the Large Hadron Collider (LHC) energies, using different medium evolution models. In Chapter 2 two of the major findings regarding radiative energy loss are discussed: • realistic medium transverse expansion suppresses elliptic flow v2 significantly because (D)GLV energy loss biases for later time interaction, when the medium is more azimuthally symmetric; • covariant frame-independent treatment of (D)GLV energy loss increases v2 by about 50%, significantly improving simultaneous agreement with experimental data on the nuclear modification factor RAA and elliptic flow v2. This is because covariance leads to an interplay between the jet propagation direction and the collective flow of medium, causing an enhanced in-plane versus out-of-plane energy loss difference. In Chapter 3 elastic energy loss is studied in detail, and two main conclusions are drawn: • compared to original TG elastic energy loss, covariant TG energy loss increases v2, especially at relatively low transverse momentum (pT); and • compared to purely radiative (D)GLV energy loss, purely elastic TG energy loss exhibits a half as large v2. After detailed discussion of radiative and elastic energy loss, Chapter 4 presents results with both radiative and elastic energy loss included, and shows that the combined elastic+radiative energy loss results in a significantly smaller v2 than purely radiative energy loss. This is true even after parameters are adjusted to reproduce the same RAA(pT) for neutral pions (π0). Chapter 5 investigates the event-by-event correlation between jet flow and medium geometry. High correlations between the jet elliptic flow v2 and the initial medium eccentricity ϵ2, as well as between the jet and the medium elliptic flow, are demonstrated
Self-Consistent Conversion of a Viscous Fluid to Particles and Heavy-Ion Physics Applications
No full textThe most widely used theoretical framework to model the early stages of a heavy-ion collision is viscous hydrodynamics. Comparing hydrodynamic simulations to heavy-ion data inevitably requires the conversion of the fluid to particles. This conversion, typically done in the Cooper-Frye formalism, is ambiguous for viscous fluids. In this thesis work, self-consistent phase space corrections are calculated by solving the linearized Boltzmann equation. These species-dependent solutions are contrasted with those obtained using the ad-hoc \u27\u27democratic Grad\u27\u27 ansatz typically employed in the literature in which coefficients are independent of particle dynamics. Solutions are calculated analytically for a massless gas and numerically for the general case of a hadron resonance gas. For example, it is found that for a gas of massless particles interacting via isotropic, energy-independent 2 → 2 scatterings, the shear viscous corrections variationally prefer a momentum dependence close to p3/2 rather than the quadratic dependence assumed in the Grad ansatz. The self-consistent phase space distributions are then used to calculate transverse momentum spectra and differential flow coefficients, v n(pT), to study the effects on heavy-ion identified particle observables. Using additive quark model cross sections, it is found that proton flow coefficients are higher than those for pions at moderately high pT in Pb + Pb collisions at LHC, especially for the coefficients v 4 and v6
Parallelized Radiative Transport and Phase Space Distributions in Heavy Ion Collisions
No full textNumerical solutions of the Boltzmann transport equation (BTE) present a framework for modeling non-equilibrium dynamics in heavy ion collisions. However, the computational power required to solve the seven-dimensional integro-differential equation reaches impractical levels for realistic, high-statistics simulations involving radiative 2 to 3 and 3 to 2 scattering processes with sequential (single-processor) algorithms. This thesis presents a new parallelized MPC/Grid code that was developed to enable such simulations. The code was tested extensively for correctness, and speedups of up to about 30x were seen relative to single-processor execution. The parallelized code was then used in a study that required high-statistic simulations, to address the ambiguity in the conversion from a fluid dynamical description to a particle description of a system. Such conversion is necessary in all comparisons of hydrodynamic simulation results to experimental data. Four existing fluid-to-particle conversion models for shear viscous fluids were assessed based on their ability to reconstruct, using hydrodynamic variables alone, the full transport phase space density for a massless one-component gas undergoing 2 to 2 scatterings in a 0+1D boost-invariant Bjorken scenario. Besides establishing the regions of validity of the four models, novel improvements are proposed that greatly increase the reconstruction accuracy of these models (by about 10x relative to the most commonly used model). Analytical simplifications of the BTE in the near-free-streaming regime are also presented, in order to gain insight into the functional form of phase space densities in the presence of interactions. These will enable the construction of yet more accurate, theoretically well-founded fluid-to-particle conversion models in the future
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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