2,227 research outputs found
Interpolation of hard and soft dilepton rates
Strict next-to-leading order (NLO) results for the dilepton production rate
from a QCD plasma at temperatures above a few hundred MeV suffer from a
breakdown of the loop expansion in the regime of soft invariant masses M^2 <<
(pi T)^2. In this regime an LPM resummation is needed for obtaining the correct
leading-order result. We show how to construct an interpolation between the
hard NLO and the leading-order LPM expression. The final numerical results are
presented in a tabulated form, suitable for insertion into hydrodynamical
codes.Comment: 16 pages. v2: wordings clarifie
Selection of Psychophysiological Features across Subjects for Classifying Workload Using Artificial Neural Networks
The issue of pilot workload is important to the United States Air Force because pilot overload or task saturation leads to decreases in mission effectiveness. Additionally, in the most extreme cases, pilot overload may lead to the loss of aircraft and crewmember lives. Current research efforts are utilizing psychophysiological data including electroencephalography (EEG), cardiac, eye-blink, and respiration measures in an attempt to identify workload levels. The primary focus of this effort is to determine if a single parsimonious set of psychophysiological features exists for accurately classifying workload levels between multiple test subjects. To accomplish this objective, the signal-to-noise (SNR) saliency measure is used to determine the usefulness of psychophysiological features in feedforward artificial neural networks (ANN). The SNR saliency measure determines the saliency, or relative value, of a feature by comparing it to a feature of injected noise. For this effort, 36 psychophysiological features were derived from the data collected as each subject completed simulated crewmember tasks using the Multi-Attribute Task Battery developed by NASA. These tasks were randomly presented to the subjects in blocks with three distinct levels: low, medium, and an overload level in which subjects could not complete all tasks
Optimization of Automatic Target Recognition with a Reject Option Using Fusion and Correlated Sensor Data
This dissertation examines the optimization of automatic target recognition (ATR) systems when a rejection option is included. First, a comprehensive review of the literature inclusive of ATR assessment, fusion, correlated sensor data, and classifier rejection is presented. An optimization framework for the fusion of multiple sensors is then developed. This framework identifies preferred fusion rules and sensors along with rejection and receiver operating characteristic (ROC) curve thresholds without the use of explicit misclassification costs as required by a Bayes\u27 loss function. This optimization framework is the first to integrate both vertical warfighter output label analysis and horizontal engineering confusion matrix analysis. In addition, optimization is performed for the true positive rate, which incorporates the time required by classification systems. The mathematical programming framework is used to assess different fusion methods and to characterize correlation effects both within and across sensors. A synthetic classifier fusion-testing environment is developed by controlling the correlation levels of generated multivariate Gaussian data. This synthetic environment is used to demonstrate the utility of the optimization framework and to assess the performance of fusion algorithms as correlation varies. The mathematical programming framework is then applied to collected radar data. This radar fusion experiment optimizes Boolean and neural network fusion rules across four levels of sensor correlation. Comparisons are presented for the maximum true positive rate and the percentage of feasible thresholds to assess system robustness. Empirical evidence suggests ATR performance may improve by reducing the correlation within and across polarimetric radar sensors. Sensitivity analysis shows ATR performance is affected by the number of forced looks, prior probabilities, the maximum allowable rejection level, and the acceptable error rates
How to compute the thermal quarkonium spectral function from first principles?
In the limit of a high temperature T and a large quark-mass M, implying a
small gauge coupling g, the heavy quark contribution to the spectral function
of the electromagnetic current can be computed systematically in the
weak-coupling expansion. We argue that the scale hierarchy relevant for
addressing the disappearance ("melting") of the resonance peak from the
spectral function reads M >> T > g^2 M > gT >> g^4 M, and review how the heavy
scales can be integrated out one-by-one, to construct a set of effective field
theories describing the low-energy dynamics. The parametric behaviour of the
melting temperature in the weak-coupling limit is specified.Comment: 8 pages; to appear in the Proceedings of SEWM08, Amsterdam, the
Netherlands, August 26-29, 200
Effects of a Thermal Bath of Photons on Embedded String Stability
We compute the corrections of thermal photons on the effective potential for
the linear sigma model of QCD. Since we are interested in temperatures lower
than the confinement temperature, we consider the scalar fields to be out of
equilibrium. Two of the scalar field are uncharged while the other two are
charged under the U(1) gauge symmetry of electromagnetism. We find that the
induced thermal terms in the effective potential can stabilize the embedded
pion string, a string configuration which is unstable in the vacuum. Our
results are applicable in a more general context and demonstrate that embedded
string configurations arising in a wider class of field theories can be
stabilized by thermal effects. Another well-known example of an embedded string
which can be stabilized by thermal effects is the electroweak Z-string. We
discuss the general criteria for thermal stabilization of embedded defects.Comment: 6 pages, formatting changed, a few typos correcte
Right-handed neutrino production rate at T > 160 GeV
The production rate of right-handed neutrinos from a Standard Model plasma at
a temperature above a hundred GeV has previously been evaluated up to NLO in
Standard Model couplings (g ~ 2/3) in relativistic (M ~ pi T) and
non-relativistic regimes (M >> pi T), and up to LO in an ultrarelativistic
regime (M < gT). The last result necessitates an all-orders resummation of the
loop expansion, accounting for multiple soft scatterings of the nearly
light-like particles participating in 1 2 reactions. In this paper we
suggest how the regimes can be interpolated into a result applicable for any
right-handed neutrino mass and at all temperatures above 160 GeV. The results
can also be used for determining the lepton number washout rate in models
containing right-handed neutrinos. Numerical results are given in a tabulated
form permitting for their incorporation into leptogenesis codes. We note that
due to effects from soft Higgs bosons there is a narrow intermediate regime
around M ~ g^{1/2} T in which our interpolation is phenomenological and a more
precise study would be welcome.Comment: 26 pages. v2: minor clarifications, published versio
Unconventional cosmology on the (thick) brane
We consider the cosmology of a thick codimension 1 brane. We obtain the
matching conditions leading to the cosmological evolution equations and show
that when one includes matter with a pressure component along the extra
dimension in the brane energy-momentum tensor, the cosmology is of non-standard
type. In particular one can get acceleration when a dust of non-relativistic
matter particles is the only source for the (modified) Friedman equation. Our
equations would seem to violate the conservation of energy-momentum from a 4D
perspective, but in 5D the energy-momentum is conserved. One could write down
an effective conserved 4D energy-momentum tensor attaching a ``dark energy''
component to the energy-momentum tensor of matter that has pressure along the
extra dimension. This extra component could, on a cosmological scale, be
interpreted as matter-coupled quintessence. We comment on the effective 4D
description of this effect in terms of the time evolution of a scalar field
(the 5D radion) coupled to this kind of matter.Comment: 9 pages, v2. eq.(17) corrected, comments on effective theory change
String Breaking in Non-Abelian Gauge Theories with Fundamental Matter Fields
We present clear numerical evidence for string breaking in three-dimensional
SU(2) gauge theory with fundamental bosonic matter through a mixing analysis
between Wilson loops and meson operators representing bound states of a static
source and a dynamical scalar. The breaking scale is calculated in the
continuum limit. In units of the lightest glueball we find . The implications of our results for QCD are discussed.Comment: 4 pages, 2 figures; equations (4)-(6) corrected, numerical results
and conclusions unchange
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