105 research outputs found
Equivalence of renormalized covariant and light-front perturbation theory: I. Longitudinal divergences in the Yukawa model
Light-front perturbation theory has been proposed as an alternative to
covariant perturbation theory. Light-front perturbation theory is only
acceptable if it produces invariant S-matrix elements. Doubts have been raised
concerning the equivalence of light-front and covariant perturbation theory.
One of the obstacles to a rigorous proof of equivalence is the occurrence of
longitudinal divergences not present in covariant perturbation theory. We show
in the case of the Yukawa model of fermions interacting with scalar bosons at
the one-loop level how to deal with the longitudinal divergences. Invariant
S-matrix elements are obtained using our method.Comment: 11 pages, epsf, revtex, contains more elaborate explanation of Forced
Instantaneous Loops (FILs
Identification of a heparin-releasable hepatic lipase binding protein from rat liver
Hepatic lipase (HL) plays a key role in the metabolism of several
lipoproteins. Metabolically active HL is bound in liver parenchymal cells
to specific binding sites. We studied the nature of the HL binding in rat
liver. Rat livers were perfused with heparin, which lead to a loss of 80%
of the HL binding capacity of the liver. The heparin-containing perfusates
possessed HL binding capacity, determined by slot-blot assay. The
perfusates were loaded on to a heparin-Sepharose column and eluted with a
linear salt gradient (0.2-1 M). HL binding activity, assessed by a
slot-blot binding assay, eluted both at 0.3 M and at 0.8 M NaCl. A 0.5 M
NaCl eluate was used to further characterize the HL binding activity. In
this fraction the major protein had a molecular mass of 70 kDa. The
fraction showed saturable HL binding in a solid-phase binding assay
Hepatic lipase is localized at the parenchymal cell microvilli in rat liver
Hepatic lipase (HL) is thought to be located at the vascular endothelium
in the liver. However, it has also been implicated in the binding and
internalization of chylomicron remnants in the parenchymal cells. In view
of this apparent discrepancy between localization and function, we
re-investigated the localization of HL in rat liver using biochemical and
immunohistochemical techniques. The binding of HL to endothelial cells was
studied in primary cultures of rat liver endothelial cells. Endothelial
cells bound HL in a saturable manner with high affinity. However, the
binding capacity accounted for at most 1% of the total HL activity present
in the whole liver. These results contrasted with earlier studies, in
which non-parenchymal cell (NPC) preparations had been found to bind HL
with a high capacity. To study HL binding to the different components of
the NPC preparations, we separated endothelial cells, Kupffer cells and
blebs by counterflow elutriation. Kupffer cells and endothelial cells
showed a relatively low HL-binding capacity. In contrast, the blebs,
representing parenchymal-cell-derived material, had a high HL-binding
capacity (33 m-units/mg of protein) and accounted for more than 80% of the
total HL binding in the NPC preparation. In contrast with endothelial and
Kupffer cells, the HL-binding capacity of parenchymal cells could account
for almost all the HL activity found in the whole liver. These data
strongly suggest that HL binding occurs at parenchymal liver cells. To
confirm this conclusion in situ, we studied HL localization by
immunocytochemical techniques. Using immunofluorescence, we confirmed the
sinusoidal localization of HL. Immunoelectron microscopy demonstrated that
virtually all HL was located at the microvilli of parenchymal liver cells,
with a minor amount at the endothelium. We conclude that, in rat liver, HL
is localized at the microvilli of parenchymal cells
Compactification near and on the light front
We address problems associated with compactification near and on the light
front. In perturbative scalar field theory we illustrate and clarify the
relationships among three approaches: (1) quantization on a space-like surface
close to a light front; (2) infinite momentum frame calculations; and (3)
quantization on the light front. Our examples emphasize the difference between
zero modes in space-like quantization and those in light front quantization. In
particular, in perturbative calculations of scalar field theory using
discretized light cone quantization there are well-known ``zero-mode induced''
interaction terms. However, we show that they decouple in the continuum limit
and covariant answers are reproduced. Thus compactification of a light-like
surface is feasible and defines a consistent field theory.Comment: 24 pages, 4 figure
The Rotation Average in Lightcone Time-Ordered Perturbation Theory
We present a rotation average of the two-body scattering amplitude in the
lightcone time()-ordered perturbation theory. Using a rotation average
procedure, we show that the contribution of individual time-ordered diagram can
be quantified in a Lorentz invariant way. The number of time-ordered diagrams
can also be reduced by half if the masses of two bodies are same. In the
numerical example of theory, we find that the higher Fock-state
contribution is quite small in the lightcone quantization.Comment: 25 pages, REVTeX, epsf.sty, 69 eps file
Infinite Nuclear Matter on the Light Front: Nucleon-Nucleon Correlations
A relativistic light front formulation of nuclear dynamics is developed and
applied to treating infinite nuclear matter in a method which includes the
correlations of pairs of nucleons: this is light front Brueckner theory. We
start with a hadronic meson-baryon Lagrangian that is consistent with chiral
symmetry. This is used to obtain a light front version of a one-boson-exchange
nucleon-nucleon potential (OBEP). The accuracy of our description of the
nucleon-nucleon (NN) data is good, and similar to that of other relativistic
OBEP models. We derive, within the light front formalism, the Hartree-Fock and
Brueckner Hartree-Fock equations. Applying our light front OBEP, the nuclear
matter saturation properties are reasonably well reproduced. We obtain a value
of the compressibility, 180 MeV, that is smaller than that of alternative
relativistic approaches to nuclear matter in which the compressibility usually
comes out too large. Because the derivation starts from a meson-baryon
Lagrangian, we are able to show that replacing the meson degrees of freedom by
a NN interaction is a consistent approximation, and the formalism allows one to
calculate corrections to this approximation in a well-organized manner. The
simplicity of the vacuum in our light front approach is an important feature in
allowing the derivations to proceed. The mesonic Fock space components of the
nuclear wave function are obtained also, and aspects of the meson and nucleon
plus-momentum distribution functions are computed. We find that there are about
0.05 excess pions per nucleon.Comment: 39 pages, RevTex, two figure
Electromagnetic form factors in the light-front formalism and the Feynman triangle diagram: spin-0 and spin-1 two-fermion systems
The connection between the Feynman triangle diagram and the light-front
formalism for spin-0 and spin-1 two-fermion systems is analyzed. It is shown
that in the limit q+ = 0 the form factors for both spin-0 and spin-1 systems
can be uniquely determined using only the good amplitudes, which are not
affected by spurious effects related to the loss of rotational covariance
present in the light-front formalism. At the same time, the unique feature of
the suppression of the pair creation process is maintained. Therefore, a
physically meaningful one-body approximation, in which all the constituents are
on their mass-shells, can be consistently formulated in the limit q+ = 0.
Moreover, it is shown that the effects of the contact term arising from the
instantaneous propagation of the active constituent can be canceled out from
the triangle diagram by means of an appropriate choice of the off-shell
behavior of the bound state vertexes; this implies that in case of good
amplitudes the Feynman triangle diagram and the one-body light-front result
match exactly. The application of our covariant light-front approach to the
evaluation of the rho-meson elastic form factors is presented.Comment: corrected typos in the reference
Light-Front Bethe-Salpeter Equation
A three-dimensional reduction of the two-particle Bethe-Salpeter equation is
proposed. The proposed reduction is in the framework of light-front dynamics.
It yields auxiliary quantities for the transition matrix and the bound state.
The arising effective interaction can be perturbatively expanded according to
the number of particles exchanged at a given light-front time. An example
suggests that the convergence of the expansion is rapid. This result is
particular for light-front dynamics. The covariant results of the
Bethe-Salpeter equation can be recovered from the corresponding auxiliary
three-dimensional ones. The technical procedure is developed for a two-boson
case; the idea for an extension to fermions is given. The technical procedure
appears quite practicable, possibly allowing one to go beyond the ladder
approximation for the solution of the Bethe-Salpeter equation. The relation
between the three-dimensional light-front reduction of the field-theoretic
Bethe-Salpeter equation and a corresponding quantum-mechanical description is
discussed.Comment: 42 pages, 5 figure
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