3,222 research outputs found
Approach to a rational rotation number in a piecewise isometric system
We study a parametric family of piecewise rotations of the torus, in the
limit in which the rotation number approaches the rational value 1/4. There is
a region of positive measure where the discontinuity set becomes dense in the
limit; we prove that in this region the area occupied by stable periodic orbits
remains positive. The main device is the construction of an induced map on a
domain with vanishing measure; this map is the product of two involutions, and
each involution preserves all its atoms. Dynamically, the composition of these
involutions represents linking together two sector maps; this dynamical system
features an orderly array of stable periodic orbits having a smooth parameter
dependence, plus irregular contributions which become negligible in the limit.Comment: LaTeX, 57 pages with 13 figure
Geometric representation of interval exchange maps over algebraic number fields
We consider the restriction of interval exchange transformations to algebraic
number fields, which leads to maps on lattices. We characterize
renormalizability arithmetically, and study its relationships with a
geometrical quantity that we call the drift vector. We exhibit some examples of
renormalizable interval exchange maps with zero and non-zero drift vector, and
carry out some investigations of their properties. In particular, we look for
evidence of the finite decomposition property: each lattice is the union of
finitely many orbits.Comment: 34 pages, 8 postscript figure
On a class of embeddings of massive Yang-Mills theory
A power-counting renormalizable model into which massive Yang-Mills theory is
embedded is analyzed. The model is invariant under a nilpotent BRST
differential s. The physical observables of the embedding theory, defined by
the cohomology classes of s in the Faddeev-Popov neutral sector, are given by
local gauge-invariant quantities constructed only from the field strength and
its covariant derivatives.Comment: LATEX, 34 pages. One reference added. Version published in the
journa
Interacting fermions and domain wall defects in 2+1 dimensions
We consider a Dirac field in 2+1 dimensions with a domain wall like defect in
its mass, minimally coupled to a dynamical Abelian vector field. The mass of
the fermionic field is assumed to have just one linear domain wall, which is
externally fixed and unaffected by the dynamics. We show that, under some
general conditions on the parameters, the localized zero modes predicted by the
Callan and Harvey mechanism are stable under the electromagnetic interaction of
the fermions
Higher-order non-symmetric counterterms in pure Yang-Mills theory
We analyze the restoration of the Slavnov-Taylor (ST) identities for pure
massless Yang-Mills theory in the Landau gauge within the BPHZL renormalization
scheme with IR regulator. We obtain the most general form of the action-like
part of the symmetric regularized action, obeying the relevant ST identities
and all other relevant symmetries of the model, to all orders in the loop
expansion. We also give a cohomological characterization of the fulfillment of
BPHZL IR power-counting criterion, guaranteeing the existence of the limit
where the IR regulator goes to zero. The technique analyzed in this paper is
needed in the study of the restoration of the ST identities for those models,
like the MSSM, where massless particles are present and no invariant
regularization scheme is known to preserve the full set of ST identities of the
theory.Comment: Final version published in the journa
Procedural Skills Training During Emergency Medicine Residency: Are We Teaching the Right Things?
Objectives: The Residency Review Committee training requirements for emergency medicine residents (EM) are defined by consensus panels, with specific topics abstracted from lists of patient complaints and diagnostic codes. The relevance of specific curricular topics to actual practice has not been studied. We compared residency graduatesâ self-assessed preparation during training to importance in practice for a variety of EM procedural skills.Methods: We distributed a web-based survey to all graduates of the Denver Health Residency Program in EM over the past 10 years. The survey addressed: practice type and patient census; years of experience; additional procedural training beyond residency; and confidence, preparation, and importance in practice for 12 procedures (extensor tendon repair, transvenous pacing, lumbar puncture, applanation tonometry, arterial line placement, anoscopy, CT scan interpretation, diagnostic peritoneal lavage, slit lamp usage, ultrasonography, compartment pressure measurement and procedural sedation). For each skill, preparation and importance were measured on four-point Likert scales. We compared mean preparation and importance scores using paired sample t-tests, to identify areas of under- or over-preparation.Results: Seventy-four residency graduates (59% of those eligible) completed the survey. There were significant discrepancies between importance in practice and preparation during residency for eight of the 12 skills. Under-preparation was significant for transvenous pacing, CT scan interpretation, slit lamp examinations and procedural sedation. Over-preparation was significant for extensor tendon repair, arterial line placement, peritoneal lavage and ultrasonography. There were strong correlations (r>0.3) between preparation during residency and confidence for 10 of the 12 procedural skills, suggesting a high degree of internal consistency for the survey.Conclusions: Practicing emergency physicians may be uniquely qualified to identify areas of under- and over-preparation during residency training. There were significant discrepancies between importance in practice and preparation during residency for eight of 12 procedures. There was a strong correlation between confidence and preparation during residency for almost all procedural skills, re-enforcing the tenet that residency training is the primary locus of instruction for clinical procedures.[WestJEM. 2009;10:152-156.
Stickiness in Hamiltonian systems: from sharply divided to hierarchical phase space
We investigate the dynamics of chaotic trajectories in simple yet physically
important Hamiltonian systems with non-hierarchical borders between regular and
chaotic regions with positive measures. We show that the stickiness to the
border of the regular regions in systems with such a sharply divided phase
space occurs through one-parameter families of marginally unstable periodic
orbits and is characterized by an exponent \gamma= 2 for the asymptotic
power-law decay of the distribution of recurrence times. Generic perturbations
lead to systems with hierarchical phase space, where the stickiness is
apparently enhanced due to the presence of infinitely many regular islands and
Cantori. In this case, we show that the distribution of recurrence times can be
composed of a sum of exponentials or a sum of power-laws, depending on the
relative contribution of the primary and secondary structures of the hierarchy.
Numerical verification of our main results are provided for area-preserving
maps, mushroom billiards, and the newly defined magnetic mushroom billiards.Comment: To appear in Phys. Rev. E. A PDF version with higher resolution
figures is available at http://www.pks.mpg.de/~edugal
A Generalized Gauge Invariant Regularization of the Schwinger Model
The Schwinger model is studied with a new one - parameter class of gauge
invariant regularizations that generalizes the usual point - splitting or
Fujikawa schemes. The spectrum is found to be qualitatively unchanged, except
for a limiting value of the regularizing parameter, where free fermions appear
in the spectrum.Comment: 16 pages, SINP/TNP/93-1
On the trace identity in a model with broken symmetry
Considering the simple chiral fermion meson model when the chiral symmetry is
explicitly broken, we show the validity of a trace identity -- to all orders of
perturbation theory -- playing the role of a Callan-Symanzik equation and which
allows us to identify directly the breaking of dilatations with the trace of
the energy-momentum tensor. More precisely, by coupling the quantum field
theory considered to a classical curved space background, represented by the
non-propagating external vielbein field, we can express the conservation of the
energy-momentum tensor through the Ward identity which characterizes the
invariance of the theory under the diffeomorphisms. Our ``Callan-Symanzik
equation'' then is the anomalous Ward identity for the trace of the
energy-momentum tensor, the so-called ``trace identity''.Comment: 11 pages, Revtex file, final version to appear in Phys.Rev.
Comment on current correlators in QCD at finite temperature
We address some criticisms by Eletsky and Ioffe on the extension of QCD sum
rules to finite temperature. We argue that this extension is possible, provided
the Operator Product Expansion and QCD-hadron duality remain valid at non-zero
temperature. We discuss evidence in support of this from QCD, and from the
exactly solvable two- dimensional sigma model O(N) in the large N limit, and
the Schwinger model.Comment: 10 pages, LATEX file, UCT-TP-208/94, April 199
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