1,902 research outputs found
Reunion of random walkers with a long range interaction: applications to polymers and quantum mechanics
We use renormalization group to calculate the reunion and survival exponents
of a set of random walkers interacting with a long range and a short
range interaction. These exponents are used to study the binding-unbinding
transition of polymers and the behavior of several quantum problems.Comment: Revtex 3.1, 9 pages (two-column format), 3 figures. Published version
(PRE 63, 051103 (2001)). Reference corrections incorporated (PRE 64, 059902
(2001) (E
Soluble `Supersymmetric' Quantum XY Model
We present a `supersymmetric' modification of the -dimensional quantum
rotor model whose ground state is exactly soluble. The model undergoes a
vortex-binding transition from insulator to metal as the rotor coupling is
varied. The Hamiltonian contains three-site terms which are relevant: they
change the universality class of the transition from that of the ()--- to
the -dimensional classical XY model. The metallic phase has algebraic ODLRO
but the superfluid density is identically zero. Variational wave functions for
single-particle and collective excitations are presented.Comment: 12 pages, REVTEX 3.0, IUCM93-00
Reconfigurable Autonomy
This position paper describes ongoing work at the Universities of Liverpool, Sheffield and Surrey in the UK on developing hybrid agent architectures for controlling autonomous systems, and specifically for ensuring that agent-controlled dynamic reconfiguration is viable. The work outlined here forms part of the Reconfigurable Autonomy research project
Off-Diagonal Long Range Order and Scaling in a Disordered Quantum Hall System
We have numerically studied the bosonic off-diagonal long range order,
introduced by Read to describe the ordering in ideal quantum Hall states, for
noninteracting electrons in random potentials confined to the lowest Landau
level. We find that it also describes the ordering in disordered quantum Hall
states: the proposed order parameter vanishes in the disordered
() phase and increases continuously from zero in the ordered
() phase. We study the scaling of the order parameter and
find that it is consistent with that of the one-electron Green's function.Comment: 10 pages and 4 figures, Revtex v3.0, UIUC preprint P-94-03-02
The high temperature expansion of the classical chain
We present the -expansion of the Helmholtz free energy of the
classical model, with a single-ion anisotropy term and in the presence of
an external magnetic field, up to order . We compare our results to
the numerical solution of Joyce's [Phys. Rev. Lett. 19, 581 (1967)] expression
for the thermodynamics of the classical model, with neither single-ion
anisotropy term nor external magnetic field. This comparison shows that the
derived analytical expansion is valid for intermediate temperatures such as
. We show that the specific heat and magnetic
susceptibility of the spin-2 antiferromagnetic chain can be approximated by
their respective classical results, up to , within an error
of 2.5%. In the absence of an external magnetic field, the ferromagnetic and
antiferromagnetic chains have the same classical Helmholtz free energy. We show
how this two types of media react to the presence of an external magnetic
field
Identifying critical source areas using multiple methods for effective diffuse pollution mitigation
Diffuse pollution from agriculture constitutes a key pressure on the water quality of freshwaters and is frequently the cause of ecological degradation. The problem of diffuse pollution can be conceptualised with a source-mobilisation-pathway (or delivery)-impact model, whereby the combination of high source risk and strong connected pathways leads to ‘critical source areas’ (CSAs). These areas are where most diffuse pollution will originate, and hence are the optimal places to implement mitigation measures. However, identifying the locations of these areas is a key problem across different spatial scales within catchments. A number of approaches are frequently used for this assessment, although comparisons of these assessments are rarely carried out. We evaluate the CSAs identified via traditional walkover surveys supported by three different approaches, highlighting their benefits and disadvantages. These include a custom designed smartphone app; a desktop geographic information system (GIS) and terrain analysis-based SCIMAP (Sensitive Catchment Integrated Modelling and Analysis Platform) approach; and the use of a high spatial resolution drone dataset as an improved input data for SCIMAP modelling. Each of these methods captures the locations of the CSAs, revealing similarities and differences in the prioritisation of CSA features. The differences are due to the temporal and spatial resolution of the three methods such as the use of static land cover information, the ability to capture small scale features, such as gateways and the incomplete catchment coverage of the walkover survey. The relative costs and output resolutions of the three methods indicate that they are suitable for application at different catchment scales in conjunction with other methods. Based on the results in this paper, it is recommended that a multi-evidence-based approach to diffuse pollution management is taken across catchment spatial scales, incorporating local knowledge from the walkover with the different data resolutions of the SCIMAP approach
Soft lepton-flavor violation in a multi-Higgs-doublet seesaw model
We consider the Standard Model with an arbitrary number n_H of Higgs doublets
and enlarge the lepton sector by adding to each lepton family \ell a
right-handed neutrino singlet \nu_{\ell R}. We assume that all Yukawa-coupling
matrices are diagonal, but the Majorana mass matrix M_R of the right-handed
neutrino singlets is an arbitrary symmetric matrix, thereby introducing an
explicit but soft violation of all lepton numbers. We investigate
lepton-flavor-violating processes within this model. We pay particular
attention to the large-m_R behavior of the amplitudes for these processes,
where m_R is the order of magnitude of the matrix elements of M_R. While the
amplitudes for processes like tau^- --> mu^- gamma and Z --> tau^+ mu^- drop as
1/m_R^2 for arbitrary n_H, processes like tau^- --> mu^- e^+ e^- and mu^- -->
e^- e^+ e^- obey this power law only for n_H = 1. For n_H \geq 2, on the
contrary, those amplitudes do not fall off when m_R increases, rather they
converge towards constants. This non-decoupling of the right-handed scale
occurs because of the sub-process ell^- --> ell'^- {S_b^0}^*, where S_b^0 is a
neutral scalar which subsequently decays to e^+ e^-. That sub-process has a
contribution from charged-scalar exchange which, for n_H \geq 2, does not
decrease when m_R tends to infinity. We also perform a general study of the
non-decoupling and argue that, after performing the limit m_R --> \infty and
removing the \nu_R from the Lagrangian, our model becomes a multi-Higgs-doublet
Standard Model with suppressed flavor-changing Yukawa couplings. Finally, we
show that, with the usual assumptions about the mass scales in the seesaw
mechanism, the branching ratios of all lepton-flavor-changing processes are
several orders of magnitude smaller than present experimental limits.Comment: 46 pages, 2 figures, Revte
Measuring the aspect ratio renormalization of anisotropic-lattice gluons
Using tadpole inproved actions we investigate the consistency between
different methods of measuring the aspect ratio renormalization of
anisotropic-lattice gluons for bare aspect ratios \chi_0=4,6,10 and inverse
lattice spacing in the range a_s^{-1}=660-840 MeV. The tadpole corrections to
the action, which are established self-consistently, are defined for two cases,
mean link tadpoles in Landau gauge and gauge invariant mean plaquette tadpoles.
Parameters in the latter case exhibited no dependence on the spatial lattice
size, L, while in the former, parameters showed only a weak dependence on L
easily extrapolated to L=\infty.
The renormalized anisotropy \chi_R was measured using both the torelon
dispersion relation and the sideways potential method. We found good agreement
between these different approaches. Any discrepancy was at worst 3-4% which is
consistent with the effect of lattice artifacts that for the torelon we
estimate as O(\a_Sa_s^2/R^2) where R is the flux-tube radius.
We also present some new data that suggests that rotational invariance is
established more accurately for the mean-link action than the plaquette action.Comment: LaTeX 18 pages including 7 figure
Heavy pseudoscalar mesons in a Schwinger-Dyson--Bethe-Salpeter approach
The mass spectrum of heavy pseudoscalar mesons, described as quark-antiquark
bound systems, is considered within the Bethe-Salpeter formalism with
momentum-dependent masses of the constituents. This dependence is found by
solving the Schwinger-Dyson equation for quark propagators in rainbow-ladder
approximation. Such an approximation is known to provide both a fast
convergence of numerical methods and accurate results for lightest mesons.
However, as the meson mass increases, the method becomes less stable and
special attention must be devoted to details of numerical means of solving the
corresponding equations. We focus on the pseudoscalar sector and show that our
numerical scheme describes fairly accurately the , , , and
ground states. Excited states are considered as well. Our calculations
are directly related to the future physics programme at FAIR.Comment: 9 pages, 3 figures; Based on materials of the contribution
"Relativistic Description of Two- and Three-Body Systems in Nuclear Physics",
ECT*, October 19-23, 200
The Virtual International Stroke Trials Archive
BACKGROUND AND PURPOSE: Stroke has global importance and it causes an increasing amount of human suffering and economic burden, but its management is far from optimal. The unsuccessful outcome of several research programs highlights the need for reliable data on which to plan future clinical trials. The Virtual International Stroke Trials Archive aims to aid the planning of clinical trials by collating and providing access to a rich resource of patient data to perform exploratory analyses.
METHODS: Data were contributed by the principal investigators of numerous trials from the past 16 years. These data have been centrally collated and are available for anonymized analysis and hypothesis testing.
RESULTS: Currently, the Virtual International Stroke Trials Archive contains 21 trials. There are data on \u3e15,000 patients with both ischemic and hemorrhagic stroke. Ages range between 18 and 103 years, with a mean age of 69+/-12 years. Outcome measures include the Barthel Index, Scandinavian Stroke Scale, National Institutes of Health Stroke Scale, Orgogozo Scale, and modified Rankin Scale. Medical history and onset-to-treatment time are readily available, and computed tomography lesion data are available for selected trials.
CONCLUSIONS: This resource has the potential to influence clinical trial design and implementation through data analyses that inform planning
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