21 research outputs found
Light-Cone Representation of the Spin and Orbital Angular Momentum of Relativistic Composite Systems
The matrix elements of local operators such as the electromagnetic current,
the energy momentum tensor, angular momentum, and the moments of structure
functions have exact representations in terms of light-cone Fock state
wavefunctions of bound states such as hadrons. We illustrate all of these
properties by giving explicit light-cone wavefunctions for the two-particle
Fock state of the electron in QED, thus connecting the Schwinger anomalous
magnetic moment to the spin and orbital momentum carried by its Fock state
constituents. We also compute the QED one-loop radiative corrections for the
form factors for the graviton coupling to the electron and photon. Although the
underlying model is derived from elementary QED perturbative couplings, it in
fact can be used to simulate much more general bound state systems by applying
spectral integration over the constituent masses while preserving all of the
Lorentz properties, giving explicit realization of the spin sum rules and other
local matrix elements. The role of orbital angular momentum in understanding
the "spin crisis" problem for relativistic systems is clarified. We also prove
that the anomalous gravitomagnetic moment B(0) vanishes for any composite
system. This property is shown to follow directly from the Lorentz boost
properties of the light-cone Fock representation and holds separately for each
Fock state component. We show how the QED perturbative structure can be used to
model bound state systems while preserving all Lorentz properties. We thus
obtain a theoretical laboratory to test the consistency of formulae which have
been proposed to probe the spin structure of hadrons.Comment: Version to be published in Nuclear Physics B. Includes illustrations
of graviton-lepton form factors at one loop in QE
On the Anomalous Discrete Symmetry
We examine an interesting scenario to solve the domain wall problem recently
suggested by Preskill, Trivedi, Wilczek and Wise. The effective potential is
calculated in the presence of the QCD axial anomaly. It is shown that some
discrete symmetries such as CP and Z_2 can be anomalous due to a so-called
-term induced by instantons. We point out that Z_2 domain-wall problem in
the two-doublet standard model can be resolved by two types of solutions: the
CP-conserving one and the CP-breaking one. In the first case, there exist two
Z_2-related local minima whose energy splitting is provided by the instanton
effect. In the second case, there is only one unique vacuum so that the domain
walls do not form at all. The consequences of this new source of CP violation
are discussed and shown to be well within the experimental limits in weak
interactions.Comment: 10 papges in LaTeX, SFU-Preprint-92-
On the nonperturbative solution of Pauli--Villars-regulated light-front QED: A comparison of the sector-dependent and standard parameterizations
We consider quantum electrodynamics quantized on the light front in Feynman
gauge and regulated in the ultraviolet by the inclusion of massive,
negative-metric Pauli--Villars (PV) particles in the Lagrangian. The eigenstate
of the electron is approximated by a Fock-state expansion truncated to include
one photon. The Fock-state wave functions are computed from the fundamental
Hamiltonian eigenvalue problem and used to calculate the anomalous magnetic
moment, as a point of comparison. Two approaches are considered: a
sector-dependent parameterization, where the bare parameters of the Lagrangian
are allowed to depend on the Fock sectors between which the particular
Hamiltonian term acts, and the standard choice, where the bare parameters are
the same for all sectors. Both methods are shown to require some care with
respect to ultraviolet divergences; neither method can allow all PV masses to
be taken to infinity. In addition, the sector-dependent approach suffers from
an infrared divergence that requires a nonzero photon mass; due to
complications associated with this divergence, the standard parameterization is
to be preferred. We also show that the self-energy effects obtained from a
two-photon truncation are enough to bring the standard-parameterization result
for the anomalous moment into agreement with experiment within numerical
errors. This continues the development of a method for the nonperturbative
solution of strongly coupled theories, in particular quantum chromodynamics.Comment: 28 pages, 2 figures, LaTeX with elsarticle.cl
Physics at the Light-Front
The light-front representation of quantum chromodynamics provides a
frame-independent, quantum-mechanical representation of hadrons at the
amplitude level, capable of encoding their multi-quark, hidden-color and gluon
momentum, helicity, and flavor correlations in the form of universal
process-independent hadron wavefunctions. The universality and
frame-independence of the LCWF's thus allow a profound connection between
diffractive dissociation, hard scattering exclusive processes such as elastic
form factors, two-photon reactions, and heavy hadron decays. In this concluding
talk of the ECT* International Conference On Light-Cone Physics: Particles And
Strings (Trento 2001), I review recent calculations and new applications of
light-front wavefunctions in QCD and other theories. I also review the
distinction between the structure functions measured in deep inelastic lepton
scattering and the quark distributions determined from light-front
wavefunctions.Comment: Invited talk presented at the International Light-Cone Workshop
"Light-cone Physics: Particles and Strings" at ECT* in Trento, Italy,
September 3-11, 200
Hadron structure from lattice quantum chromodynamics
This is a review of hadron structure physics from lattice QCD. Throughout
this report, we place emphasis on the contribution of lattice results to our
understanding of a number of fundamental physics questions related to, e.g.,
the origin and distribution of the charge, magnetization, momentum and spin of
hadrons. Following an introduction to some of the most important hadron
structure observables, we summarize the methods and techniques employed for
their calculation in lattice QCD. We briefly discuss the status of relevant
chiral perturbation theory calculations needed for controlled extrapolations of
the lattice results to the physical point. In the main part of this report, we
give an overview of lattice calculations on hadron form factors, moments of
(generalized) parton distributions, moments of hadron distribution amplitudes,
and other important hadron structure observables. Whenever applicable, we
compare with results from experiment and phenomenology, taking into account
systematic uncertainties in the lattice computations. Finally, we discuss
promising results based on new approaches, ideas and techniques, and close with
remarks on future perspectives of the field.Comment: 189 pages, to be published in Physics Report