2,892 research outputs found
Classical and quantum many-body description of bremsstrahlung in dense matter (Landau - Pomeranchuk - Migdal effect)
Some considerations about the importance of coherence effects for
bremsstrahlung processes in non--equilibrium dense matter (Landau - Pomeranchuk
- Migdal - effect) are presented. They are of particular relevance for the
application to photon - and di-lepton production from high energy nuclear
collisions, to gluon radiation in QCD transport, or parton kinetics and to
neutrino and axion radiation from supernova explosion and from hot neutron
stars. The soft behavior of the bremsstrahlung from a source described by
classical transport models is discussed and pocket correction formulas for the
in-matter radiation cross sections are suggested in terms of standard transport
coefficients. The radiation rates are also discussed within a non--equilibrium
quantum field theory (Schwinger - Kadanoff - Baym - Keldysh) formulation. A
classification of diagrams and corresponding resummation in physically
meaningful terms is proposed, which considers the finite damping width of all
source particles in matter. This way each diagram in this expansion is already
free from the infra--red divergences. Both, the correct quasi--particle and
quasi--classical limits are recovered from this subset of graphs. Explicit
results are given for dense matter in thermal equilibrium. The diagrammatic
description may suggest a formulation of a transport theory that includes the
propagation of off--shell particles in non--equilibrium dense matter.Comment: 50 pages, submitted to Ann. Phys. (N. Y.); diagrams coded as
tex-macros; 5 figures available at:
ftp://tpri6b.gsi.de/pub/knoll/ap-95-fig.uu; paper as postscript file
(compressed and uuencoded) available at:
ftp://tpri6b.gsi.de/pub/knoll/ap-95.p
NON-EQUILIBRIUM DESCRIPTION OF BREMSSTRAHLUNG IN DENSE MATTER (Landau - Pomeranchuk - Migdal Effect)
The soft behavior of the bremsstrahlung from a source is discussed in terms
of classical transport models and within a non--equilibrium quantum field
theory (Schwinger - Kadanoff - Baym - Keldysh) formulation.Comment: 8 pages, latex, no figures (diagrams in latex
Soft Modes, Resonances and Quantum Transport
Effects of the propagation of particles, which have a finite life-time and an
according width in their mass spectrum, are discussed in the context of
transport description. First, the importance of coherence effects
(Landau-Pomeranchuk-Migdal effect) on production and absorption of field quanta
in non-equilibrium dense matter is considered. It is shown that classical
diffusion and Langevin results correspond to re-summation of certain
field-theory diagrams formulated in terms of full non-equilibrium Green's
functions. Then the general properties of broad resonances in dense and hot
systems are discussed in the framework of a self-consistent and conserving
Phi-derivable method of Baym at the examples of the rho-meson in hadronic
matter and the pion in dilute nuclear matter. Further we address the problem of
a transport description that properly accounts for the damping width of the
particles. The Phi-derivable method generalized to the real-time contour
provides a self-consistent and conserving kinetic scheme. We derive a
generalized expression for the non-equilibrium kinetic entropy flow, which
includes corrections from fluctuations and mass-width effects. In special cases
an H-theorem is proved. Memory effects in collision terms give contributions to
the kinetic entropy flow that in the Fermi-liquid case recover the famous
bosonic type T^3 ln T correction to the specific heat of liquid Helium-3. At
the example of the pion-condensate phase transition in dense nuclear matter we
demonstrate important part played by the width effects within the quantum
transport.Comment: submitted to Phys. At. Nucl. (Rus.), the volume dedicated to the
memory of A.B. Migdal. 31 pages, 5 figure
Evaluation of initial collector field performance at the Langley Solar Building Test Facility
The thermal performance of the solar collector field for the NASA Langley Solar Building Test Facility is given for October 1976 through January 1977. A 1,180 square meter solar collector field with seven collector designs helped to provide hot water for the building heating system and absorption air conditioner. The collectors were arranged in 12 rows with nominally 51 collectors per row. Heat transfer rates for each row were calculated and recorded along with sensor, insolation, and weather data every five minutes using a minicomputer. The agreement between the experimental and predicted collector efficiencies was generally within five percentage points
Design, development and test of shuttle/Centaur G-prime cryogenic tankage thermal protection systems
The thermal protection systems for the shuttle/Centaur would have had to provide fail-safe thermal protection during prelaunch, launch ascent, and on-orbit operations as well as during potential abort. The thermal protection systems selected used a helium-purged polyimide foam beneath three rediation shields for the liquid-hydrogen tank and radiation shields only for the liquid-oxygen tank (three shields on the tank sidewall and four on the aft bulkhead). A double-walled vacuum bulkhead separated the two tanks. The liquid-hydrogen tank had one 0.75-in-thick layer of foam on the forward bulkhead and two layers on the larger area sidewall. Full scale tests of the flight vehicle in a simulated shuttle cargo bay that was purged with gaseous nitrogen gave total prelaunch heating rates of 88,500 Btu/hr and 44,000 Btu/hr for the liquid-hydrogen and -oxygen tanks, respectively. Calorimeter tests on a representative sample of the liquid-hydrogen tank sidewall thermal protection system indicated that the measured unit heating rate would rapidly decrease from the prelaunch rate of approx 100 Btu/hr/sq ft to a desired rate of less than 1.3 Btu/hr/sq ft once on orbit
Soft Modes, Quantum Transport and Kinetic Entropy
The effects of the propagation of particles which have a finite life-time and
an according width in their mass spectrum are discussed in the context of
transport descriptions. In the first part the coupling of soft photon modes to
a source of charged particles is studied in a classical model which can be
solved completely in analytical terms. The solution corresponds to a
re-summation of certain field theory diagrams. The general properties of broad
resonances in dense finite temperature systems are discussed at the example of
the -meson in hadronic matter. The second part addresses the problem of
transport descriptions which also account for the damping width of the
particles. The Kadanoff--Baym equation after gradient approximation together
with the -derivable method of Baym provides a self-consistent and
conserving scheme. Memory effects appearing in collision term diagrams of
higher order are discussed. We derive a generalized expression for the
nonequilibrium kinetic entropy flow, which includes corrections from
fluctuations and mass-width effects. In special cases an -theorem is proved.
Memory effects in collision terms provide contributions to the kinetic entropy
flow that in the Fermi-liquid case recover the famous bosonic type
correction to the specific heat of liquid Helium-3.Comment: Contribution to Proc. of Int. Workshop "Kadanoff-Baym Equations -
Progress and Perspectives for Many-Body Physics" Rostock (Germany), September
20-24 1999, ed. M.Bonitz, World Scientific (2000
Initial operation of a solar heating and cooling system in a full-scale solar building test facility
The Solar Building Test Facility (SBTF) was constructed to advance the technology for heating and cooling of office buildings with solar energy. Its purposes are to (1) test system components which include high-performing collectors, (2) test the performance of a complete solar heating and cooling system, (3) investigate component interactions, and (4) investigate durability, maintenance and reliability of components. The SBTF consists of a 50,000 square foot office building modified to accept solar heated water for operation of an absorption air conditioner and for the baseboard heating system. A 12,666 square foot solar collector field with a 30,000 gallon storage tank provides the solar heated water. A description of the system and the collectors selected is printed along with the objectives, test approach, expected system performance, and some preliminary results
Renormalization in Self-Consistent Approximations schemes at Finite Temperature I: Theory
Within finite temperature field theory, we show that truncated
non-perturbative self-consistent Dyson resummation schemes can be renormalized
with local counter-terms defined at the vacuum level. The requirements are that
the underlying theory is renormalizable and that the self-consistent scheme
follows Baym''s -derivable concept. The scheme generates both, the
renormalized self-consistent equations of motion and the closed equations for
the infinite set of counter terms. At the same time the corresponding
2PI-generating functional and the thermodynamical potential can be
renormalized, in consistency with the equations of motion. This guarantees the
standard -derivable properties like thermodynamic consistency and exact
conservation laws also for the renormalized approximation schemes to hold. The
proof uses the techniques of BPHZ-renormalization to cope with the explicit and
the hidden overlapping vacuum divergences.Comment: 22 Pages 1 figure, uses RevTeX4. The Revision concerns the correction
of some minor typos, a clarification concerning the real-time contour
structure of renormalization parts and some comments concerning symmetries in
the conclusions and outloo
A brief intervention increases fruit and vegetable intake: a comparison of two intervention sequences
Background and Purpose: To evaluate the effectiveness of two subsequent intervention components (motivational and self-regulatory components), placed in different order, to promote fruit and vegetable (FV) intake. Methods: After baseline assessment, university students (N = 205, aged 18-26 years) were allocated to two groups. One group received a motivational intervention (outcome expectancies, risk perception, and task self-efficacy) followed by a self-regulatory intervention (planning and dietary self-efficacy) after 17 days. The second group received the same intervention conditions in the opposite order. Follow-up assessments were done after another 17 days. Results: Both intervention sequences yielded gains in terms of FV intake and self-efficacy. However, this gain was only due to the self-regulatory component whereas the motivational component did not contribute to the changes. Moreover, changes in intention and self-efficacy mediated between intervention sequence and follow-up behavior, suggesting that improving these proximal predictors of FV intake was responsible for the behavioral gains. Conclusions: Findings highlight the superiority of a self-regulatory intervention over a motivational intervention when it comes to dietary changes in this sample of young adults. Moreover, changes in dietary self-efficacy may drive nutritional changes.info:eu-repo/semantics/publishedVersio
- …