51,859 research outputs found
Lattice theory for nonrelativistic fermions in one spatial dimension
I derive a loop representation for the canonical and grand-canonical
partition functions for an interacting four-component Fermi gas in one spatial
dimension and an arbitrary external potential. The representation is free of
the "sign problem" irrespective of population imbalance, mass imbalance, and to
a degree, sign of the interaction strength. This property is in sharp contrast
with the analogous three-dimensional two-component interacting Fermi gas, which
exhibits a sign problem in the case of unequal masses, chemical potentials, and
repulsive interactions. The one-dimensional system is believed to exhibit many
phenomena in common with its three-dimensional counterpart, including an analog
of the BCS-BEC crossover, and nonperturbative universal few- and many-body
physics at scattering lengths much larger than the range of interaction, making
the theory an interesting candidate for numerical study. Positivity of the
probability measure for the partition function allows for a mean-field
treatment of the model; here, I present such an analysis for the interacting
Fermi gas in the SU(4) (unpolarized, mass-symmetric) limit, and demonstrate
that there exists a phase in which a continuum limit may be defined.Comment: 12 pages, 6 figures, references adde
Approximate solutions for the single soliton in a Skyrmion-type model with a dilaton scalar field
We consider the analytical properties of the single-soliton solution in a
Skyrmion-type Lagrangian that incorporates the scaling properties of quantum
chromodynamics (QCD) through the coupling of the chiral field to a scalar field
interpreted as a bound state of gluons. The model was proposed in previous
works to describe the Goldstone pions in a dense medium, being also useful for
studying the properties of nuclear matter and the in-medium properties of
mesons and nucleons. Guided by an asymptotic analysis of the Euler-Lagrange
equations, we propose approximate analytical representations for the single
soliton solution in terms of rational approximants exponentially localized.
Following the Pad\'e method, we construct a sequence of approximants from the
exact power series solutions near the origin. We find that the convergence of
the approximate representations to the numerical solutions is considerably
improved by taking the expansion coefficients as free parameters and then
minimizing the mass of the Skyrmion using our ans\"atze for the fields. We also
perform an analysis of convergence by computation of physical quantities
showing that the proposed analytical representations are very useful useful for
phenomenological calculations.Comment: 13 pages, 3 eps figures, version to be published in Phys.Rev.
Exploring degeneracies in modified gravity with weak lensing
By considering linear-order departures from general relativity, we compute a
novel expression for the weak lensing convergence power spectrum under
alternative theories of gravity. This comprises an integral over a 'kernel' of
general relativistic quantities multiplied by a theory-dependent 'source' term.
The clear separation between theory-independent and -dependent terms allows for
an explicit understanding of each physical effect introduced by altering the
theory of gravity. We take advantage of this to explore the degeneracies
between gravitational parameters in weak lensing observations.Comment: 17 pages, 7 figures. v2: Minor changes to match version accepted by
PR
Vector Meson Dominance as a first step in a systematic approximation: the pion vector form factor
Pade Approximants can be used to go beyond Vector Meson Dominance in a
systematic approximation. We illustrate this fact with the case of the pion
vector form factor and extract values for the first two coefficients of its
Taylor expansion. Pade Approximants are shown to be a useful and simple tool
for incorporating high-energy information, allowing an improved determination
of these Taylor coefficients.Comment: 13 pages, 7 figure
Towards understanding Regge trajectories in holographic QCD
We reassess a work done by Migdal on the spectrum of low-energy vector mesons
in QCD in the light of the AdS-QCD correspondence. Recently, a tantalizing
parallelism was suggested between Migdal's work and a family of holographic
duals of QCD. Despite the intriguing similarities, both approaches face a major
drawback: the spectrum is in conflict with well-tested Regge scaling. However,
it has recently been shown that holographic duals can be modified to accomodate
Regge behavior. Therefore, it is interesting to understand whether Regge
behavior can also be achieved in Migdal's approach. In this paper we
investigate this issue. We find that Migdal's approach, which is based on a
modified Pade approximant, is closely related to the issue of quark-hadron
duality breakdown in QCD.Comment: 17 pages, 1 figure. Typos fixed, references added, improved
discussion. Minor changes to match the journal versio
Integration of crosswind forces into train dynamic modelling
In this paper a new method is used to calculate unsteady wind loadings acting on a railway vehicle. The method takes input data from wind tunnel testing or from computational fluid dynamics simulations (one example of each is presented in this article), for aerodynamic force and moment coefficients and combines these with fluctuating wind velocity time histories and train speed to produce wind force time histories on the train. This method is fast and efficient and this has allowed the wind forces to be applied to a vehicle dynamics simulation for a long length of track.
Two typical vehicles (one passenger, one freight) have been modelled using the vehicle dynamics simulation package ‘VAMPIRE®’, which allows detailed modelling of the vehicle suspension and wheel—rail contact. The aerodynamic coefficients of the passenger train have been obtained from wind tunnel tests while those of the freight train have been obtained through fluid dynamic computations using large-eddy simulation. Wind loadings were calculated for the same vehicles for a range of average wind speeds and applied to the vehicle models using a user routine within the VAMPIRE package. Track irregularities measured by a track recording coach for a 40 km section of the main line route from London to King's Lynn were used as input to the vehicle simulations.
The simulated vehicle behaviour was assessed against two key indicators for derailment; the Y/Q ratio, which is an indicator of wheel climb derailment, and the Δ Q/Q value, which indicates wheel unloading and therefore potential roll over. The results show that vehicle derailment by either indicator is not predicted for either vehicle for any mean wind speed up to 20 m/s (with consequent gusts up to around 30 m/s). At a higher mean wind speed of 25 m/s derailment is predicted for the passenger vehicle and the unladen freight vehicle (but not for the laden freight vehicle)
The Chemical Evolution of the Universe I: High Column Density Absorbers
We construct a simple, robust model of the chemical evolution of galaxies
from high to low redshift, and apply it to published observations of damped
Lyman-alpha quasar absorption line systems (DLAs). The elementary model assumes
quiescent star formation and isolated galaxies (no interactions, mergers or gas
flows). We consider the influence of dust and chemical gradients in the
galaxies, and hence explore the selection effects in quasar surveys. We fit
individual DLA systems to predict some observable properties of the absorbing
galaxies, and also indicate the expected redshift behaviour of chemical element
ratios involving nucleosynthetic time delays.
Despite its simplicity, our `monolithic collapse' model gives a good account
of the distribution and evolution of the metallicity and column density of
DLAs, and of the evolution of the global star formation rate and gas density
below redshifts z 3. However, from the comparison of DLA observations with our
model, it is clear that star formation rates at higher redshifts (z>3) are
enhanced. Galaxy interactions and mergers, and gas flows very probably play a
major role.Comment: 36 pages, 11 figures; accepted by MNRA
Integration of tools for the Design and Assessment of High-Performance, Highly Reliable Computing Systems (DAHPHRS), phase 1
Systems for Space Defense Initiative (SDI) space applications typically require both high performance and very high reliability. These requirements present the systems engineer evaluating such systems with the extremely difficult problem of conducting performance and reliability trade-offs over large design spaces. A controlled development process supported by appropriate automated tools must be used to assure that the system will meet design objectives. This report describes an investigation of methods, tools, and techniques necessary to support performance and reliability modeling for SDI systems development. Models of the JPL Hypercubes, the Encore Multimax, and the C.S. Draper Lab Fault-Tolerant Parallel Processor (FTPP) parallel-computing architectures using candidate SDI weapons-to-target assignment algorithms as workloads were built and analyzed as a means of identifying the necessary system models, how the models interact, and what experiments and analyses should be performed. As a result of this effort, weaknesses in the existing methods and tools were revealed and capabilities that will be required for both individual tools and an integrated toolset were identified
Analysis of the X-Factor and X-Factor stretch during the completion of a golf practice session in low-handicap golfers
The X-Factor and X-Factor stretch have been positively correlated with golf long game performance. The aim of this study was to compare the X-Factor, X-Factor stretch and long game performance variables pre and following a golf practice session. A group of male golfers (n = 15, handicap = 3.3 ± 1.7) participated in the laboratory-based-study. Movement and performance variables were collected from five golf swings performed pre and following a golf practice session using a motion capture system and launch monitor respectively. Following the practice session, significant increases were observed in the X-Factor (p = 0.00, d = 0.22) and the X-Factor stretch (p = 0.02, d = 0.25). Specifically, the X-Factor increased from 52.82 ± 5.64 ° to 54.06 ± 5.61 ° following the practice session. The X-Factor stretch increased from 1.54 ± 1.05 ° to 1.90 ± 1.41 ° following the practice session. Significant differences were displayed in club head velocity (p = 0.00, d = 0.35), ball velocity (p = 0.01, d = 0.21) and actual carry distance (p = 0.00, d = 0.29) following the practice session. These findings suggest that performing multiple golf shots is not detrimental in terms of muscular fatigue in the long game performance. In actual fact, the findings demonstrate that performing 100 golf shots increases the X-Factor, X-Factor stretch patterns and performance variables which, in turn, increases long game performance. These findings can help PGA golf Professionals improve teaching practices and formulation of golf programmes and warm-up sessions
Identifying the causal mechanisms of the quiet eye
Scientists who have examined the gaze strategies employed by athletes have determined that longer quiet eye (QE) durations (QED) are characteristic of skilled compared to less-skilled performers. However, the cognitive mechanisms of the QE and, specifically, how the QED affects performance are not yet fully understood. We review research that has examined the functional mechanism underlying QE and discuss the neural networks that may be involved. We also highlight the limitations surrounding QE measurement and its definition and propose future research directions to address these shortcomings. Investigations into the behavioural and neural mechanisms of QE will aid the understanding of the perceptual and cognitive processes underlying expert performance and the factors that change as expertise develops
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