822 research outputs found
Vehicle/engine integration
VEHICLE/ENGINE Integration Issues are explored for orbit transfer vehicles (OTV's). The impact of space basing and aeroassist on VEHICLE/ENGINE integration is discussed. The AOTV structure and thermal protection subsystem weights were scaled as the vehicle length and surface was changed. It is concluded that for increased allowable payload lengths in a ground-based system, lower length-to-diameter (L/D) is as important as higher mixture ration (MR) in the range of mid L/D ATOV's. Scenario validity, geometry constraints, throttle levels, reliability, and servicing are discussed in the context of engine design and engine/vehicle integration
Suite of simple metrics reveals common movement syndromes across vertebrate taxa
ecause empirical studies of animal movement are most-often site- and species-specific, we lack understanding of the level of consistency in movement patterns across diverse taxa, as well as a framework for quantitatively classifying movement patterns. We aim to address this gap by determining the extent to which statistical signatures of animal movement patterns recur across ecological systems. We assessed a suite of movement metrics derived from GPS trajectories of thirteen marine and terrestrial vertebrate species spanning three taxonomic classes, orders of magnitude in body size, and modes of movement (swimming, flying, walking). Using these metrics, we performed a principal components analysis and cluster analysis to determine if individuals organized into statistically distinct clusters. Finally, to identify and interpret commonalities within clusters, we compared them to computer-simulated idealized movement syndromes representing suites of correlated movement traits observed across taxa (migration, nomadism, territoriality, and central place foraging)
Neutrino self-energy and dispersion in a medium with magnetic field
We calculate the one-loop thermal self-energy of a neutrino in a constant and
homogeneous magnetic field to all orders in the magnetic field strength using
Schwinger's proper time method. We obtain the dispersion relation under various
conditions.Comment: 17 pp, RevTeX, one figur
Quark Dispersion Relation and Dilepton Production in the Quark-Gluon Plasma
Under very general assumptions we show that the quark dispersion relation in
the quark-gluon plasma is given by two collective branches, of which one has a
minimum at a non-vanishing momentum. This general feature of the quark
dispersion relation leads to structures (van Hove singularities, gaps) in the
low mass dilepton production rate, which might provide a unique signature for
the quark-gluon plasma formation in relativistic heavy ion collisions.Comment: 6 pages, Revtex, 2 PostScript figures, revised version to be
published in Phys. Rev. Let
Can Van Hove singularities be observed in relativistic heavy-ion collisions ?
Based on general arguments the in-medium quark propagator in a quark-gluon
plasma leads to a quark dispersion relation consisting of two branches, of
which one exhibits a minimum at some finite momentum. This results in a
vanishing group velocity for collective quark modes. Important quantities such
as the production rate of low mass lepton pairs and mesonic correlators depend
inversely on this group velocity. Therefore these quantities, which follow from
self energy diagrams containing a quark loop, are strongly affected by Van Hove
singularities (peaks and gaps). If these sharp structures could be observed in
relativistic heavy-ion collisions it would reveal the physical picture of the
QGP as a gas of quasiparticles.Comment: 12 pages including nine figures and style files, invited talk given
at the ICPAQGP-2001, November 26-30, 2001, Jaipur, Indi
Nonequilibrium perturbation theory for spin-1/2 fields
A partial resummation of perturbation theory is described for field theories
containing spin-1/2 particles in states that may be far from thermal
equilibrium. This allows the nonequilibrium state to be characterized in terms
of quasiparticles that approximate its true elementary excitations. In
particular, the quasiparticles have dispersion relations that differ from those
of free particles, finite thermal widths and occupation numbers which, in
contrast to those of standard perturbation theory evolve with the changing
nonequilibrium environment. A description of this kind is essential for
estimating the evolution of the system over extended periods of time. In
contrast to the corresponding description of scalar particles, the structure of
nonequilibrium fermion propagators exhibits features which have no counterpart
in the equilibrium theory.Comment: 16 pages; no figures; submitted to Phys. Rev.
Dynamical Mass Generation in a Finite-Temperature Abelian Gauge Theory
We write down the gap equation for the fermion self-energy in a
finite-temperature abelian gauge theory in three dimensions. The instantaneous
approximation is relaxed, momentum-dependent fermion and photon self-energies
are considered, and the corresponding Schwinger-Dyson equation is solved
numerically. The relation between the zero-momentum and zero-temperature
fermion self-energy and the critical temperature T_c, above which there is no
dynamical mass generation, is then studied. We also investigate the effect
which the number of fermion flavours N_f has on the results, and we give the
phase diagram of the theory with respect to T and N_f.Comment: 20 LaTeX pages, 4 postscript figures in a single file, version to
appear in Physical Review
Quantum Dissipation and Decay in Medium
Quantum dissipation in thermal environment is investigated, using the path
integral approach. The reduced density matrix of the harmonic oscillator system
coupled to thermal bath of oscillators is derived for arbitrary spectrum of
bath oscillators. Time evolution and the end point of two-body decay of
unstable particles is then elucidated: After early transient times unstable
particles undergo the exponential decay, followed by the power law decay and
finally ending in a mixed state of residual particles containing contributions
from both on and off the mass shell, whose abundance does not suffer from the
Boltzmann suppression.Comment: 19 pages, LATEX file. Substantially expanded and revised for
publication, including more complete description of application to unstable
particle decay in thermal medium. Some minor mistake of numerical factors
correcte
Axino dark matter from thermal production
The axino is a promising candidate for dark matter in the Universe. It is
electrically and color neutral, very weakly interacting, and could be - as
assumed in this study - the lightest supersymmetric particle, which is stable
for unbroken R-parity. In supersymmetric extensions of the standard model, in
which the strong CP problem is solved via the Peccei-Quinn mechanism, the axino
arises naturally as the fermionic superpartner of the axion. We compute the
thermal production rate of axinos in supersymmetric QCD. Using hard thermal
loop resummation, we obtain a finite result in a gauge-invariant way, which
takes into account Debye screening in the hot quark-gluon-squark-gluino plasma.
The relic axino abundance from thermal scatterings after inflation is
evaluated. We find that thermally produced axinos could provide the dominant
part of cold dark matter, for example, for an axino mass of 100 keV and a
reheating temperature of 10^6 GeV.Comment: 33 pages, 7 figures, 1 table, erratum adde
Exact Effective Action for (1+1 Dimensional) Fermions in an Abelian Background at Finite Temperature
In an effort to further understand the structure of effective actions for
fermions in an external gauge background at finite temperature, we study the
example of 1+1 dimensional fermions interacting with an arbitrary Abelian gauge
field. We evaluate the effective action exactly at finite temperature. This
effective action is non-analytic as is expected at finite temperature. However,
contrary to the structure at zero temperature and contrary to naive
expectations, the effective action at finite temperature has interactions to
all (even) orders (which, however, do not lead to any quantum corrections). The
covariant structure thus obtained may prove useful in studying 2+1 dimensional
models in arbitrary backgrounds. We also comment briefly on the solubility of
various 1+1 dimensional models at finite temperature.Comment: A few clarifying remarks added;21 page
- …