2,701 research outputs found
The supersymmetric Ward identities on the lattice
Supersymmetric (SUSY) Ward identities are considered for the N=1 SU(2) SUSY
Yang Mills theory discretized on the lattice with Wilson fermions (gluinos).
They are used in order to compute non-perturbatively a subtracted gluino mass
and the mixing coefficient of the SUSY current. The computations were performed
at gauge coupling =2.3 and hopping parameter =0.1925, 0.194,
0.1955 using the two-step multi-bosonic dynamical-fermion algorithm. Our
results are consistent with a scenario where the Ward identities are satisfied
up to O(a) effects. The vanishing of the gluino mass occurs at a value of the
hopping parameter which is not fully consistent with the estimate based on the
chiral phase transition. This suggests that, although SUSY restoration appears
to occur close to the continuum limit of the lattice theory, the results are
still affected by significant systematic effects.Comment: 34 pages, 7 figures. Typo corrected, last sentence reformulated,
reference added. To appear in Eur. Phys. J.
Counterflow Extension for the F.A.S.T.-Model
The F.A.S.T. (Floor field and Agent based Simulation Tool) model is a
microscopic model of pedestrian dynamics, which is discrete in space and time.
It was developed in a number of more or less consecutive steps from a simple CA
model. This contribution is a summary of a study on an extension of the
F.A.S.T-model for counterflow situations. The extensions will be explained and
it will be shown that the extended F.A.S.T.-model is capable of handling
various counterflow situations and to reproduce the well known lane formation
effect.Comment: Contribution to Crowds and Cellular Automata Workshop 2008. Accepted
for publication in "Cellular Automata -- 8th International Conference on
Cellular Automata for Research and Industry, ACRI 2008, Yokohama, Japan,
September 23-26, Springer 2008, Proceedings
Recommended from our members
Upland streamwater nitrate dynamics across decadal to sub-daily timescales: a case study of Plynlimon, Wales
Streamwater nitrate dynamics in the River Hafren, Plynlimon, mid-Wales were investigated over decadal to sub-daily timescales using a range of statistical techniques. Long-term data were derived from weekly grab samples (1984–2010) and high-frequency data from 7-hourly samples (2007–2009) both measured at two sites: a headwater stream draining moorland and a downstream site below plantation forest. This study is one of the first to analyse upland streamwater nitrate dynamics across such a wide range of timescales and report on the principal mechanisms identified. The data analysis provided no clear evidence that the long-term decline in streamwater nitrate concentrations was related to a decline in atmospheric deposition alone, because nitrogen deposition first increased and then decreased during the study period. Increased streamwater temperature and denitrification may also have contributed to the decline in stream nitrate concentrations, the former through increased N uptake rates and the latter resultant from increased dissolved organic carbon concentrations. Strong seasonal cycles, with concentration minimums in the summer, were driven by seasonal flow minimums and seasonal biological activity enhancing nitrate uptake. Complex diurnal dynamics were observed, with seasonal changes in phase and amplitude of the cycling, and the diurnal dynamics were variable along the river. At the moorland site, a regular daily cycle, with minimum concentrations in the early afternoon, corresponding with peak air temperatures, indicated the importance of instream biological processing. At the downstream site, the diurnal dynamics were a composite signal, resultant from advection, dispersion and nitrate processing in the soils of the lower catchment. The diurnal streamwater nitrate dynamics were also affected by drought conditions. Enhanced diurnal cycling in Spring 2007 was attributed to increased nitrate availability in the post-drought period as well as low flow rates and high temperatures over this period. The combination of high-frequency short-term measurements and long-term monitoring provides a powerful tool for increasing understanding of the controls of element fluxes and concentrations in surface waters
Analytical approximation for single-impurity Anderson model
We have applied the recently developed dual fermion technique to the spectral
properties of single-band Anderson impurity problem (SIAM). In our approach a
series expansion is constructed in vertices of the corresponding atomic
Hamiltonian problem. This expansion contains a small parameter in two limiting
cases: in the weak coupling case (), due to the smallness of the
irreducible vertices, and near the atomic limit (), when bare
propagators are small. Reasonable results are obtained also for the most
interesting case of strong correlations (). The atomic problem of
the Anderson impurity model has a degenerate ground state, so the application
of the perturbation theory is not straightforward. We construct a special
approach dealing with symmetry-broken ground state of the renormalized atomic
problem. Formulae for the first-order dual diagram correction are obtained
analytically in the real-time domain. Most of the Kondo-physics is reproduced:
logarithmic contributions to the self energy arise, Kondo-like peak at the
Fermi level appears, and the Friedel sum rule is fulfilled. Our approach
describes also renormalization of atomic resonances due to hybridization with a
conduction band. A generalization of the proposed scheme to a multi-orbital
case can be important for the realistic description of correlated solids.Comment: 6 pages, 5 figure
A new approach to spherically symmetric junction surfaces and the matching of FLRW regions
We investigate timelike junctions (with surface layer) between spherically
symmetric solutions of the Einstein-field equation. In contrast to previous
investigations this is done in a coordinate system in which the junction
surface motion is absorbed in the metric, while all coordinates are continuous
at the junction surface.
The evolution equations for all relevant quantities are derived. We discuss
the no-surface layer case (boundary surface) and study the behaviour for small
surface energies. It is shown that one should expect cases in which the speed
of light is reached within a finite proper time.
We carefully discuss necessary and sufficient conditions for a possible
matching of spherically symmetric sections.
For timelike junctions between spherically symmetric space-time sections we
show explicitly that the time component of the Lanczos equation always reduces
to an identity (independently of the surface equation of state).
The results are applied to the matching of FLRW models. We discuss `vacuum
bubbles' and closed-open junctions in detail. As illustrations several
numerical integration results are presented, some of them indicate that the
junction surface can reach the speed of light within a finite time.Comment: new version - corrected boundary surface discussion, improved
presentation, and corrected reference 22 pages, many figure
Conserving Diagrammatic Approximations for Quantum Impurity Models: NCA and CTMA
Self-consistent diagrammatic approximations to the Anderson or Kondo impurity
model, using an exact pseudoparticle representation of the impurity states, are
reviewed. We first discuss the infrared exponents of the pseudoparticle
propagators as indicators of Fermi liquid behavior through their dependence on
the impurity occupation and on magnetic field. Then we discuss the Non-Crossing
Approximation (NCA), identifying its strengths, but also its fundamental
shortcomings. Physical arguments as well as a perturbative renormalization
group analysis suggest that an infinite parquet-type resummation of
two-particle vertex diagrams, the Conserving T-Matrix Approximation (CTMA) will
cure the deficiencies of NCA. We review results on the pseudoparticle spectral
functions, the spin susceptibility and the impurity electron spectral function,
supporting that the CTMA provides qualitatively correct results, both in the
high-temperature regime and in the strong coupling Fermi liquid regime at low
temperatures.Comment: 10 pages, invited article, to appear in a special edition of JPSJ
"Kondo Effect - 40 Years after the Discovery", published version, some minor
typos correcte
Beta-delayed deuteron emission from 11Li: decay of the halo
The deuteron-emission channel in the beta-decay of the halo-nucleus 11Li was
measured at the ISAC facility at TRIUMF by implanting post-accelerated 11Li
ions into a segmented silicon detector. The events of interest were identified
by correlating the decays of 11Li with those of the daughter nuclei. This
method allowed the energy spectrum of the emitted deuterons to be extracted,
free from contributions from other channels, and a precise value for the
branching ratio B_d = 1.30(13) x 10-4 to be deduced for E(c.m.) > 200 keV. The
results provide the first unambiguous experimental evidence that the decay
takes place essentially in the halo of 11Li, and that it proceeds mainly to the
9Li + d continuum, opening up a new means to study of the halo wave function of
11Li.Comment: 4 pages, 3 figure
Transport Properties of One-Dimensional Hubbard Models
We present results for the zero and finite temperature Drude weight D(T) and
for the Meissner fraction of the attractive and the repulsive Hubbard model, as
well as for the model with next nearest neighbor repulsion. They are based on
Quantum Monte Carlo studies and on the Bethe ansatz. We show that the Drude
weight is well defined as an extrapolation on the imaginary frequency axis,
even for finite temperature. The temperature, filling, and system size
dependence of D is obtained. We find counterexamples to a conjectured
connection of dissipationless transport and integrability of lattice models.Comment: 10 pages, 14 figures. Published versio
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