1,757 research outputs found
Thirteen category recognition map of Yellowstone National Park produced from ERTS-1 MSS data
There are no author-identified significant results in this report
Transmission of an arenavirus in white-throated woodrats (Neotoma albigula), southeastern Colorado, 1995-1999.
From 1995 to 1999, we conducted longitudinal studies of white- throated woodrats (Neotoma albigula) in southeastern Colorado. Forty-five (42.9%) of 105 female and 15 (26.8%) of 56 male N. albigula had antibodies against Whitewater Arroyo virus (WWAV). Sixteen female and three male N. albigula seroconverted during the study period, most of them during July-November, when population densities are highest. Analyses of longevity data, minimum numbers alive and infected, movements, and weight data suggest that the dominant mode of WWAV transmission among white-throated woodrats in Colorado is direct contact. WWAV was recently reported to cause fatal infection in humans. Our findings will lead to better assessment of the public health threat posed by infected woodrats and may be useful in predicting periods of increased risk for human infection
Infrared Behaviour of Systems With Goldstone Bosons
We develop various complementary concepts and techniques for handling quantum
fluctuations of Goldstone bosons.We emphasise that one of the consequences of
the masslessness of Goldstone bosons is that the longitudinal fluctuations also
have a diverging susceptibility characterised by an anomalous dimension
in space-time dimensions .In these fluctuations diverge
logarithmically in the infrared region.We show the generality of this
phenomenon by providing three arguments based on i). Renormalization group
flows, ii). Ward identities, and iii). Schwinger-Dyson equations.We obtain an
explicit form for the generating functional of one-particle irreducible
vertices of the O(N) (non)--linear --models in the leading 1/N
approximation.We show that this incorporates all infrared behaviour correctly
both in linear and non-linear -- models. Our techniques provide an
alternative to chiral perturbation theory.Some consequences are discussed
briefly.Comment: 28 pages,2 Figs, a new section on some universal features of
multipion processes has been adde
Exact and approximate dynamics of the quantum mechanical O(N) model
We study a quantum dynamical system of N, O(N) symmetric, nonlinear
oscillators as a toy model to investigate the systematics of a 1/N expansion.
The closed time path (CTP) formalism melded with an expansion in 1/N is used to
derive time evolution equations valid to order 1/N (next-to-leading order). The
effective potential is also obtained to this order and its properties
areelucidated. In order to compare theoretical predictions against numerical
solutions of the time-dependent Schrodinger equation, we consider two initial
conditions consistent with O(N) symmetry, one of them a quantum roll, the other
a wave packet initially to one side of the potential minimum, whose center has
all coordinates equal. For the case of the quantum roll we map out the domain
of validity of the large-N expansion. We discuss unitarity violation in the 1/N
expansion; a well-known problem faced by moment truncation techniques. The 1/N
results, both static and dynamic, are also compared to those given by the
Hartree variational ansatz at given values of N. We conclude that late-time
behavior, where nonlinear effects are significant, is not well-described by
either approximation.Comment: 16 pages, 12 figrures, revte
Antigenic Complementarity in the Origins of Autoimmunity: A General Theory Illustrated With a Case Study of Idiopathic Thrombocytopenia Purpura
We describe a novel, testable theory of autoimmunity, outline novel predictions made by the theory, and illustrate its application to unravelling the possible causes of idiopathic thrombocytopenia purpura (ITP). Pairs of stereochemically complementary antigens induce complementary immune responses (antibody or T-cell) that create loss of regulation and civil war within the immune system itself. Antibodies attack antibodies creating circulating immune complexes; T-cells attack T-cells creating perivascular cuffing. This immunological civil war abrogates the self-nonself distinction. If at least one of the complementary antigens mimics a self antigen, then this unregulated immune response will target host tissues as well. Data demonstrating that complementary antigens are found in some animal models of autoimmunity and may be present in various human diseases, especially ITP, are reviewed. Specific mechanisms for preventing autoimmunity or suppressing existing autoimmunity are derived from the theory, and critical tests proposed. Finally, we argue that Koch's postulates are inadequate for establishing disease causation for multiple-antigen diseases and discuss the possibility that current research has failed to elucidate the causes of human autoimmune diseases because we are using the wrong criteria
Thermal Density Functional Theory in Context
This chapter introduces thermal density functional theory, starting from the
ground-state theory and assuming a background in quantum mechanics and
statistical mechanics. We review the foundations of density functional theory
(DFT) by illustrating some of its key reformulations. The basics of DFT for
thermal ensembles are explained in this context, as are tools useful for
analysis and development of approximations. We close by discussing some key
ideas relating thermal DFT and the ground state. This review emphasizes thermal
DFT's strengths as a consistent and general framework.Comment: Submitted to Spring Verlag as chapter in "Computational Challenges in
Warm Dense Matter", F. Graziani et al. ed
Non-Equilibrium Quantum Fields in the Large N Expansion
An effective action technique for the time evolution of a closed system
consisting of one or more mean fields interacting with their quantum
fluctuations is presented. By marrying large expansion methods to the
Schwinger-Keldysh closed time path (CTP) formulation of the quantum effective
action, causality of the resulting equations of motion is ensured and a
systematic, energy conserving and gauge invariant expansion about the
quasi-classical mean field(s) in powers of developed. The general method
is exposed in two specific examples, symmetric scalar \l\F^4 theory
and Quantum Electrodynamics (QED) with fermion fields. The \l\F^4 case is
well suited to the numerical study of the real time dynamics of phase
transitions characterized by a scalar order parameter. In QED the technique may
be used to study the quantum non-equilibrium effects of pair creation in strong
electric fields and the scattering and transport processes in a relativistic
plasma. A simple renormalization scheme that makes practical the
numerical solution of the equations of motion of these and other field theories
is described.Comment: 43 pages, LA-UR-94-783 (PRD, in press), uuencoded PostScrip
Defect Formation and Critical Dynamics in the Early Universe
We study the nonequilibrium dynamics leading to the formation of topological
defects in a symmetry-breaking phase transition of a quantum scalar field with
\lambda\Phi^4 self-interaction in a spatially flat, radiation-dominated
Friedmann-Robertson-Walker Universe. The quantum field is initially in a
finite-temperature symmetry-restored state and the phase transition develops as
the Universe expands and cools. We present a first-principles, microscopic
approach in which the nonperturbative, nonequilibrium dynamics of the quantum
field is derived from the two-loop, two-particle-irreducible closed-time-path
effective action. We numerically solve the dynamical equations for the
two-point function and we identify signatures of topological defects in the
infrared portion of the momentum-space power spectrum. We find that the density
of topological defects formed after the phase transition scales as a power law
with the expansion rate of the Universe. We calculate the equilibrium critical
exponents of the correlation length and relaxation time for this model and show
that the power law exponent of the defect density, for both overdamped and
underdamped evolution, is in good agreement with the "freeze-out" scenario of
Zurek. We introduce an analytic dynamical model, valid near the critical point,
that exhibits the same power law scaling of the defect density with the quench
rate. By incorporating the realistic quench of the expanding Universe, our
approach illuminates the dynamical mechanisms important for topological defect
formation. The observed power law scaling of the defect density with the quench
rate, observered here in a quantum field theory context, provides evidence for
the "freeze-out" scenario in three spatial dimensions.Comment: 31 pages, RevTex, 8 figures in EPS forma
The next to leading order effective potential in the 2+1 dimensional Nambu-Jona-Lasinio model at finite temperature
The finite temperature effective potential in the 2+1 dimensional
Nambu-Jona-Lasinio model is constructed up to the next to leading order in the
large expansion, where is the number of flavors in the model. The
distinctive feature of the analysis is an inclusion of an additional scalar
field, which allows us to circumvent the well known, and otherwise unavoidable
problem with the imaginary contribution to the effective potential. In
accordance with the Mermin-Wagner-Coleman theorem, applied to the dimensionally
reduced subsystem of the zero Matsubara modes of the composite boson fields,
the finite temperature effective potential reveals a global minimum at the zero
of the composite order parameter. This allows us to conclude that the
continuous global symmetry of the NJL model is not broken for any arbitrarily
small, finite temperature.Comment: 12 pages, 4 figures, REVTe
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