47 research outputs found
Shear Viscosity in a Perturbative Quark-Gluon-Plasma
Among the key features of hot and dense QCD matter produced in
ultra-relativistic heavy-ion collisions at RHIC is its very low shear
viscosity, indicative of the properties of a near-ideal fluid, and a large
opacity demonstrated by jet energy loss measurements. In this work, we utilize
a microscopic transport model based on the Boltzmann equation with quark and
gluon degrees of freedom and cross sections calculated from perturbative
Quantum Chromodynamics to simulate an ideal Quark-Gluon-Plasma in full thermal
and chemical equilibrium. We then use the Kubo formalism to calculate the shear
viscosity to entropy density ratio of the medium as a function of temperature
and system composition. One of our key results is that the shear viscosity over
entropy-density ratio becomes invariant to the chemical composition of
the system when plotted as a function of energy-density instead of temperature.Comment: 11 pages, 8 figures: version #2 contains some revisions and added
references to clarify relationship to previously published wor
Relativistic shock waves in viscous gluon matter
We solve the relativistic Riemann problem in viscous gluon matter employing a
microscopic parton cascade. We demonstrate the transition from ideal to viscous
shock waves by varying the shear viscosity to entropy density ratio
from zero to infinity. We show that an ratio larger than 0.2 prevents
the development of well-defined shock waves on timescales typical for
ultrarelativistic heavy-ion collisions. Comparisons with viscous hydrodynamic
calculations confirm our findings.Comment: Version as published in PRL 103, 032301 (2009). 4 pages, 4 figure
Investigation of shock waves in the relativistic Riemann problem: A comparison of viscous fluid dynamics to kinetic theory
We solve the relativistic Riemann problem in viscous matter using the
relativistic Boltzmann equation and the relativistic causal dissipative
fluid-dynamical approach of Israel and Stewart. Comparisons between these two
approaches clarify and point out the regime of validity of second-order fluid
dynamics in relativistic shock phenomena. The transition from ideal to viscous
shocks is demonstrated by varying the shear viscosity to entropy density ratio
. We also find that a good agreement between these two approaches
requires a Knudsen number .Comment: Version as published in PRC 82, 024910 (2010); 16 pages, 16 figures,
typos correcte
Transport Theoretical Description of Collisional Energy Loss in Infinite Quark-Gluon Matter
We study the time evolution of a high-momentum gluon or quark propagating
through an infinite, thermalized, partonic medium utilizing a Boltzmann
equation approach. We calculate the collisional energy loss of the parton,
study its temperature and flavor dependence as well as the the momentum
broadening incurred through multiple interactions. Our transport calculations
agree well with analytic calculations of collisional energy-loss where
available, but offer the unique opportunity to address the medium response as
well in a consistent fashion.Comment: 12 pages, updated with additional references and typos correcte
Multiplicity Distributions in Canonical and Microcanonical Statistical Ensembles
The aim of this paper is to introduce a new technique for calculation of
observables, in particular multiplicity distributions, in various statistical
ensembles at finite volume. The method is based on Fourier analysis of the
grand canonical partition function. Taylor expansion of the generating function
is used to separate contributions to the partition function in their power in
volume. We employ Laplace's asymptotic expansion to show that any equilibrium
distribution of multiplicity, charge, energy, etc. tends to a multivariate
normal distribution in the thermodynamic limit. Gram-Charlier expansion allows
additionally for calculation of finite volume corrections. Analytical formulas
are presented for inclusion of resonance decay and finite acceptance effects
directly into the system partition function. This paper consolidates and
extends previously published results of current investigation into properties
of statistical ensembles.Comment: 53 pages, 7 figure
Comparative Genomics of 2009 Seasonal Plague (Yersinia pestis) in New Mexico
Plague disease caused by the Gram-negative bacterium Yersinia pestis routinely affects animals and occasionally humans, in the western United States. The strains native to the North American continent are thought to be derived from a single introduction in the late 19th century. The degree to which these isolates have diverged genetically since their introduction is not clear, and new genomic markers to assay the diversity of North American plague are highly desired. To assay genetic diversity of plague isolates within confined geographic areas, draft genome sequences were generated by 454 pyrosequencing from nine environmental and clinical plague isolates. In silico assemblies of Variable Number Tandem Repeat (VNTR) loci were compared to laboratory-generated profiles for seven markers. High-confidence SNPs and small Insertion/Deletions (Indels) were compared to previously sequenced Y. pestis isolates. The resulting panel of mutations allowed clustering of the strains and tracing of the most likely evolutionary trajectory of the plague strains. The sequences also allowed the identification of new putative SNPs that differentiate the 2009 isolates from previously sequenced plague strains and from each other. In addition, new insertion points for the abundant insertion sequences (IS) of Y. pestis are present that allow additional discrimination of strains; several of these new insertions potentially inactivate genes implicated in virulence. These sequences enable whole-genome phylogenetic analysis and allow the unbiased comparison of closely related isolates of a genetically monomorphic pathogen
Nearly Perfect Fluidity: From Cold Atomic Gases to Hot Quark Gluon Plasmas
Shear viscosity is a measure of the amount of dissipation in a simple fluid.
In kinetic theory shear viscosity is related to the rate of momentum transport
by quasi-particles, and the uncertainty relation suggests that the ratio of
shear viscosity eta to entropy density s in units of hbar/k_B is bounded by a
constant. Here, hbar is Planck's constant and k_B is Boltzmann's constant. A
specific bound has been proposed on the basis of string theory where, for a
large class of theories, one can show that eta/s is greater or equal to hbar/(4
pi k_B). We will refer to a fluid that saturates the string theory bound as a
perfect fluid. In this review we summarize theoretical and experimental
information on the properties of the three main classes of quantum fluids that
are known to have values of eta/s that are smaller than hbar/k_B. These fluids
are strongly coupled Bose fluids, in particular liquid helium, strongly
correlated ultracold Fermi gases, and the quark gluon plasma. We discuss the
main theoretical approaches to transport properties of these fluids: kinetic
theory, numerical simulations based on linear response theory, and holographic
dualities. We also summarize the experimental situation, in particular with
regard to the observation of hydrodynamic behavior in ultracold Fermi gases and
the quark gluon plasma.Comment: 76 pages, 11 figures, review article, extensive revision
Pathosphere.org: pathogen detection and characterization through a web-based, open source informatics platform
Background
The detection of pathogens in complex sample backgrounds has been revolutionized by wide access to next-generation sequencing (NGS) platforms. However, analytical methods to support NGS platforms are not as uniformly available. Pathosphere (found at Pathosphere.org) is a cloud - based open - sourced community tool that allows for communication, collaboration and sharing of NGS analytical tools and data amongst scientists working in academia, industry and government. The architecture allows for users to upload data and run available bioinformatics pipelines without the need for onsite processing hardware or technical support.
Results
The pathogen detection capabilities hosted on Pathosphere were tested by analyzing pathogen-containing samples sequenced by NGS with both spiked human samples as well as human and zoonotic host backgrounds. Pathosphere analytical pipelines developed by Edgewood Chemical Biological Center (ECBC) identified spiked pathogens within a common sample analyzed by 454, Ion Torrent, and Illumina sequencing platforms. ECBC pipelines also correctly identified pathogens in human samples containing arenavirus in addition to animal samples containing flavivirus and coronavirus. These analytical methods were limited in the detection of sequences with limited homology to previous annotations within NCBI databases, such as parvovirus. Utilizing the pipeline-hosting adaptability of Pathosphere, the analytical suite was supplemented by analytical pipelines designed by the United States Army Medical Research Insititute of Infectious Diseases and Walter Reed Army Institute of Research (USAMRIID-WRAIR). These pipelines were implemented and detected parvovirus sequence in the sample that the ECBC iterative analysis previously failed to identify.
Conclusions
By accurately detecting pathogens in a variety of samples, this work demonstrates the utility of Pathosphere and provides a platform for utilizing, modifying and creating pipelines for a variety of NGS technologies developed to detect pathogens in complex sample backgrounds. These results serve as an exhibition for the existing pipelines and web-based interface of Pathosphere as well as the plug-in adaptability that allows for integration of newer NGS analytical software as it becomes available
QCD and strongly coupled gauge theories : challenges and perspectives
We highlight the progress, current status, and open challenges of QCD-driven physics, in theory and in experiment. We discuss how the strong interaction is intimately connected to a broad sweep of physical problems, in settings ranging from astrophysics and cosmology to strongly coupled, complex systems in particle and condensed-matter physics, as well as to searches for physics beyond the Standard Model. We also discuss how success in describing the strong interaction impacts other fields, and, in turn, how such subjects can impact studies of the strong interaction. In the course of the work we offer a perspective on the many research streams which flow into and out of QCD, as well as a vision for future developments.Peer reviewe