10,101 research outputs found
S-matrix elements for gauge theories with and without implemented constraints
We derive an expression for the relation between two scattering transition
amplitudes which reflect the same dynamics, but which differ in the description
of their initial and final state vectors. In one version, the incident and
scattered states are elements of a perturbative Fock space, and solve the
eigenvalue problem for the `free' part of the Hamiltonian --- the part that
remains after the interactions between particle excitations have been `switched
off'. Alternatively, the incident and scattered states may be coherent states
that are transforms of these Fock states. In earlier work, we reported on the
scattering amplitudes for QED, in which a unitary transformation relates
perturbative and non-perturbative sets of incident and scattered states. In
this work, we generalize this earlier result to the case of transformations
that are not necessarily unitary and that may not have unique inverses. We
discuss the implication of this relationship for Abelian and non-Abelian gauge
theories in which the `transformed', non-perturbative states implement
constraints, such as Gauss's law.Comment: 8 pages. Invited contribution to Foundation of Physics for an issue
honoring Prof. Lawrence Horwitz on his 65th Birthda
Quantum Gauge Equivalence in QED
We discuss gauge transformations in QED coupled to a charged spinor field,
and examine whether we can gauge-transform the entire formulation of the theory
from one gauge to another, so that not only the gauge and spinor fields, but
also the forms of the operator-valued Hamiltonians are transformed. The
discussion includes the covariant gauge, in which the gauge condition and
Gauss's law are not primary constraints on operator-valued quantities; it also
includes the Coulomb gauge, and the spatial axial gauge, in which the
constraints are imposed on operator-valued fields by applying the
Dirac-Bergmann procedure. We show how to transform the covariant, Coulomb and
spatial axial gauges to what we call
``common form,'' in which all particle excitation modes have identical
properties. We also show that, once that common form has been reached, QED in
different gauges has a common time-evolution operator that defines
time-translation for states that represent systems of electrons and photons.
By combining gauge transformations with changes of representation from
standard to common form, the entire apparatus of a gauge theory can be
transformed from one gauge to another.Comment: Contribution for a special issue of Foundations of Physics honoring
Fritz Rohrlich; edited by Larry P. Horwitz, Tel-Aviv University, and Alwyn
van der Merwe, University of Denver (Plenum Publishing, New York); 40 pages,
REVTEX, Preprint UCONN-93-3, 1 figure available upon request from author
Gauge equivalence in QCD: the Weyl and Coulomb gauges
The Weyl-gauge ( QCD Hamiltonian is unitarily transformed to a
representation in which it is expressed entirely in terms of gauge-invariant
quark and gluon fields. In a subspace of gauge-invariant states we have
constructed that implement the non-Abelian Gauss's law, this unitarily
transformed Weyl-gauge Hamiltonian can be further transformed and, under
appropriate circumstances, can be identified with the QCD Hamiltonian in the
Coulomb gauge. We demonstrate an isomorphism that materially facilitates the
application of this Hamiltonian to a variety of physical processes, including
the evaluation of -matrix elements. This isomorphism relates the
gauge-invariant representation of the Hamiltonian and the required set of
gauge-invariant states to a Hamiltonian of the same functional form but
dependent on ordinary unconstrained Weyl-gauge fields operating within a space
of ``standard'' perturbative states. The fact that the gauge-invariant
chromoelectric field is not hermitian has important implications for the
functional form of the Hamiltonian finally obtained. When this nonhermiticity
is taken into account, the ``extra'' vertices in Christ and Lee's Coulomb-gauge
Hamiltonian are natural outgrowths of the formalism. When this nonhermiticity
is neglected, the Hamiltonian used in the earlier work of Gribov and others
results.Comment: 25 page
Gauge-invariant fields in the temporal gauge, Coulomb-gauge fields, and the Gribov ambiguity
We examine the relation between Coulomb-gauge fields and the gauge-invariant
fields constructed in the temporal gauge for two-color QCD by comparing a
variety of properties, including their equal-time commutation rules and those
of their conjugate chromoelectric fields. We also express the temporal-gauge
Hamiltonian in terms of gauge-invariant fields and show that it can be
interpreted as a sum of the Coulomb-gauge Hamiltonian and another part that is
important for determining the equations of motion of temporal-gauge fields, but
that can never affect the time evolution of ``physical'' state vectors. We also
discuss multiplicities of gauge-invariant temporal-gauge fields that belong to
different topological sectors and that, in previous work, were shown to be
based on the same underlying gauge-dependent temporal-gauge fields. We argue
that these multiplicities of gauge-invariant fields are manifestations of the
Gribov ambiguity. We show that the differential equation that bases the
multiplicities of gauge-invariant fields on their underlying gauge-dependent
temporal-gauge fields has nonlinearities identical to those of the ``Gribov''
equation, which demonstrates the non-uniqueness of Coulomb-gauge fields. These
multiplicities of gauge-invariant fields --- and, hence, Gribov copies ---
appear in the temporal gauge, but only with the imposition of Gauss's law and
the implementation of gauge invariance; they do not arise when the theory is
represented in terms of gauge-dependent fields and Gauss's law is left
unimplemented.Comment: 27 pages, 1 figure; text has been revised and references adde
Topology of the gauge-invariant gauge field in two-color QCD
We investigate solutions to a nonlinear integral equation which has a central
role in implementing the non-Abelian Gauss's Law and in constructing
gauge-invariant quark and gluon fields. Here we concern ourselves with
solutions to this same equation that are not operator-valued, but are functions
of spatial variables and carry spatial and SU(2) indices. We obtain an
expression for the gauge-invariant gauge field in two-color QCD, define an
index that we will refer to as the ``winding number'' that characterizes it,
and show that this winding number is invariant to a small gauge transformation
of the gauge field on which our construction of the gauge-invariant gauge field
is based. We discuss the role of this gauge field in determining the winding
number of the gauge-invariant gauge field. We also show that when the winding
number of the gauge field is an integer , the gauge-invariant
gauge field manifests winding numbers that are not integers, and are
half-integers only when .Comment: 26 pages including 6 encapsulated postscript figures. Numerical
errors have been correcte
Developing Seismogenic Source Models Based on Geologic Fault Data
Calculating seismic hazard usually requires input that includes seismicity associated with
known faults, historical earthquake catalogs, geodesy, and models of ground shaking.
This paper will address the input generally derived from geologic studies that augment
the short historical catalog to predict ground shaking at time scales of tens, hundreds, or
thousands of years (e.g., SSHAC 1997). A seismogenic source model, terminology we
adopt here for a fault source model, includes explicit three-dimensional faults deemed
capable of generating ground motions of engineering significance within a specified time
frame of interest. In tectonically active regions of the world, such as near plate
boundaries, multiple seismic cycles span a few hundred to a few thousand years. In
contrast, in less active regions hundreds of kilometers from the nearest plate boundary,
seismic cycles generally are thousands to tens of thousands of years long. Therefore, one
should include sources having both longer recurrence intervals and possibly older times
of most recent rupture in less active regions of the world rather than restricting the model
to include only Holocene faults (i.e., those with evidence of large-magnitude earthquakes
in the past 11,500 years) as is the practice in tectonically active regions with high
deformation rates.
During the past 15 years, our institutions independently developed databases to
characterize seismogenic sources based on geologic data at a national scale. Our goal
here is to compare the content of these two publicly available seismogenic source models
compiled for the primary purpose of supporting seismic hazard calculations by the
Istituto Nazionale di Geofisica e Vulcanologia (INGV) and the U.S. Geological Survey
(USGS); hereinafter we refer to the two seismogenic source models as INGV and USGS,
respectively. This comparison is timely because new initiatives are emerging to
characterize seismogenic sources at the continental scale (e.g., SHARE in the Euro-
Mediterranean, http://www.share-eu.org/; EMME in the Middle East, http://www.emmegem.
org/) and global scale (e.g., GEM, http://www.globalquakemodel.org/; Anonymous
2008). To some extent, each of these efforts is still trying to resolve the level of optimal
detail required for this type of compilation. The comparison we provide defines a
common standard for consideration by the international community for future regional
and global seismogenic source models by identifying the necessary parameters that
capture the essence of geological fault data in order to characterize seismogenic sources.
In addition, we inform potential users of differences in our usage of common
geological/seismological terms to avoid inappropriate use of the data in our models and
provide guidance to convert the data from one model to the other (for detailed
instructions, see the electronic supplement to this article). Applying our recommendations
will permit probabilistic seismic hazard assessment codes to run seamlessly using either
seismogenic source input.
The USGS and INGV database schema compare well at a first-level inspection.
Both databases contain a set of fields representing generalized fault three-dimensional
geometry and additional fields that capture the essence of past earthquake occurrences.
Nevertheless, there are important differences. When we further analyze supposedly comparable fields, many are defined differently. These differences would cause
anomalous results in hazard prediction if one assumes the values are similarly defined.
The data, however, can be made fully compatible using simple transformations
Persistent Transport Barrier on the West Florida Shelf
Analysis of drifter trajectories in the Gulf of Mexico has revealed the
existence of a region on the southern portion of the West Florida Shelf (WFS)
that is not visited by drifters that are released outside of the region. This
so-called ``forbidden zone'' (FZ) suggests the existence of a persistent
cross-shelf transport barrier on the southern portion of the WFS. In this
letter a year-long record of surface currents produced by a Hybrid-Coordinate
Ocean Model simulation of the WFS is used to identify Lagrangian coherent
structures (LCSs), which reveal the presence of a robust and persistent
cross-shelf transport barrier in approximately the same location as the
boundary of the FZ. The location of the cross-shelf transport barrier undergoes
a seasonal oscillation, being closer to the coast in the summer than in the
winter. A month-long record of surface currents inferred from high-frequency
(HF) radar measurements in a roughly 60 km 80 km region on the WFS off
Tampa Bay is also used to identify LCSs, which reveal the presence of robust
transient transport barriers. While the HF-radar-derived transport barriers
cannot be unambiguously linked to the boundary of the FZ, this analysis does
demonstrate the feasibility of monitoring transport barriers on the WFS using a
HF-radar-based measurement system. The implications of a persistent cross-shelf
transport barrier on the WFS for the development of harmful algal blooms on the
shoreward side of the barrier are considered.Comment: Submitted to Geophysical Research Letter
The global electroweak fit at NNLO and prospects for the LHC and ILC
For a long time, global fits of the electroweak sector of the Standard Model
(SM) have been used to exploit measurements of electroweak precision
observables at lepton colliders (LEP, SLC), together with measurements at
hadron colliders (Tevatron, LHC), and accurate theoretical predictions at
multi-loop level, to constrain free parameters of the SM, such as the Higgs and
top masses. Today, all fundamental SM parameters entering these fits are
experimentally determined, including information on the Higgs couplings, and
the global fits are used as powerful tools to assess the validity of the theory
and to constrain scenarios for new physics. Future measurements at the Large
Hadron Collider (LHC) and the International Linear Collider (ILC) promise to
improve the experimental precision of key observables used in the fits. This
paper presents updated electroweak fit results using newest NNLO theoretical
predictions, and prospects for the LHC and ILC. The impact of experimental and
theoretical uncertainties is analysed in detail. We compare constraints from
the electroweak fit on the Higgs couplings with direct LHC measurements, and
examine present and future prospects of these constraints using a model with
modified couplings of the Higgs boson to fermions and bosons.Comment: 26 pages, 9 figure
Spin Susceptibility of Noncentrosymmetric Heavy-fermion Superconductor CeIrSi3 under Pressure: 29Si-Knight Shift Study on Single Crystal
We report 29Si-NMR study on a single crystal of the heavy-fermion
superconductor CeIrSi3 without an inversion symmetry along the c-axis. The
29Si-Knight shift measurements under pressure have revealed that the spin
susceptibility for the ab-plane decreases slightly below Tc, whereas along the
c-axis it does not change at all. The result can be accounted for by the spin
susceptibility in the superconducting state being dominated by the strong
antisymmetric (Rashba-type) spin-orbit interaction that originates from the
absence of an inversion center along the c-axis and it being much larger than
superconducting condensation energy. This is the first observation which
exhibits an anisotropy of the spin susceptibility below Tc in the
noncentrosymmetric superconductor dominated by strong Rashba-type spin-orbit
interaction.Comment: 4 pages, 4 figures, Accepted for publication in Phys. Rev. Let
Precision Measurements of Stretching and Compression in Fluid Mixing
The mixing of an impurity into a flowing fluid is an important process in
many areas of science, including geophysical processes, chemical reactors, and
microfluidic devices. In some cases, for example periodic flows, the concepts
of nonlinear dynamics provide a deep theoretical basis for understanding
mixing. Unfortunately, the building blocks of this theory, i.e. the fixed
points and invariant manifolds of the associated Poincare map, have remained
inaccessible to direct experimental study, thus limiting the insight that could
be obtained. Using precision measurements of tracer particle trajectories in a
two-dimensional fluid flow producing chaotic mixing, we directly measure the
time-dependent stretching and compression fields. These quantities, previously
available only numerically, attain local maxima along lines coinciding with the
stable and unstable manifolds, thus revealing the dynamical structures that
control mixing. Contours or level sets of a passive impurity field are found to
be aligned parallel to the lines of large compression (unstable manifolds) at
each instant. This connection appears to persist as the onset of turbulence is
approached.Comment: 5 pages, 5 figure
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