3,303 research outputs found
Walker Design for Kinetic Assessment of Upper Extremity Joint Demands in Children with Osteogenesis Imperfecta
On the nullification of threshold amplitudes
The nullification of threshold amplitudes is considered within the
conventional framework of quantum field theory. The relevant Ward identities
for the reduced theory are derived both on path-integral and diagrammatic
levels. They are then used to prove the vanishing of tree-graph threshold
amplitudes.Comment: 16 page
Normalization of QCD corrections in top quark decay
We discuss the effects of QCD corrections to the on-shell decay .
We resolve the scale ambiguity using the Brodsky-Lepage-Mackenzie scheme, and
find that the appropriate coupling constant is . The largest long distance contribution comes from the definition
of the on-shell mass of the top quark. We note that QCD corrections to the
electroweak parameter are extremely small when the parameter is
expressed in terms of the top quark width.Comment: 7 pages (LaTeX), TPI-MINN-94/16-T, UMN-TH-1252/9
Simulation and optimisation of terahertz emission from InGaAs and InP photoconductive switches
We simulate the terahertz emission from laterally-biased InGaAs and InP using
a three-dimensional carrier dynamics model in order to optimise the
semiconductor material. Incident pump-pulse parameters of current Ti:Sapphire
and Er:fibre lasers are chosen, and the simulation models the semiconductor's
bandstructure using parabolic Gamma, L and X valleys, and heavy holes. The
emitted terahertz radiation is propagated within the semiconductor and into
free space using a model based on the Drude-Lorentz dielectric function. As the
InGaAs alloy approaches InAs an increase in the emitted power is observed, and
this is attributed to a greater electron mobility. Additionally,
low-temperature grown and ion-implanted InGaAs are modelled using a finite
carrier trapping time. At sub-picosecond trapping times the terahertz bandwidth
is found to increase significantly at the cost of a reduced emission power.Comment: 9 pages, 7 figure
E+ E- -> Tau+ Tau- at the Threshold and Beyond
The excitation curve for the production in electron
positron annihilation near the threshold is revisited with the aim of updating
and extending a previous work. We find that the corrections of the relative
magnitude near the threshold are significantly contributed by the
radiative modification of the Coulomb interaction between the leptons.
The interpolation between the Coulomb effects at the threshold and the
relativistic effects at higher energies is considered and the resulting formula
is argued to have relative accuracy up to, but not including, terms of the
order of at any velocity of the leptons.Comment: 7 pages, 1 figure (LaTeX, figure appended in PostScript),
UMN-TH-1232/93, TPI-MINN-93/61-
Testing refinements by refining tests
One of the potential benefits of formal methods is that they offer the possibility of reducing the costs of testing. A specification acts as both the benchmark against which any implementation is tested, and also as the means by which tests are generated. There has therefore been interest in developing test generation techniques from formal specifications, and a number of different methods have been derived for state based languages such as Z, B and VDM. However, in addition to deriving tests from a formal specification, we might wish to refine the specification further before its implementation. The purpose of this paper is to explore the relationship between testing and refinement. As our model for test generation we use a DNF partition analysis for operations written in Z, which produces a number of disjoint test cases for each operation. In this paper we discuss how the partition analysis of an operation alters upon refinement, and we develop techniques that allow us to refine abstract tests in order to generate test cases for a refinement. To do so we use (and extend existing) methods for calculating the weakest data refinement of a specification
On the Strong Coupling Limit of the Faddeev-Hopf Model
The variational calculus for the Faddeev-Hopf model on a general Riemannian
domain, with general Kaehler target space, is studied in the strong coupling
limit. In this limit, the model has key similarities with pure Yang-Mills
theory, namely conformal invariance in dimension 4 and an infinite dimensional
symmetry group. The first and second variation formulae are calculated and
several examples of stable solutions are obtained. In particular, it is proved
that all immersive solutions are stable. Topological lower energy bounds are
found in dimensions 2 and 4. An explicit description of the spectral behaviour
of the Hopf map S^3 -> S^2 is given, and a conjecture of Ward concerning the
stability of this map in the full Faddeev-Hopf model is proved.Comment: 21 pages, 0 figure
The onset of e^+e^- -> tau^+ tau^- at threshold revisited
The precise knowledge of the onset of the cross section at the threshold is necessary for improving the accuracy of
determination of the mass from the threshold measurements. The QED
radiative corrections of relative order and , additional to
the well known Coulomb factor, are considered in the threshold region. The
correction terms of order are calculated, which contain coefficients
enhanced by large parameters. As a result it is argued that the known
corrections provide the accuracy of the description of the cross
section close to , rather than as claimed in a recent
literature. Also analytical expressions are provided for some limiting cases of
the corrections, previously calculated numerically.Comment: 16 page
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