1,510 research outputs found
Extended Iterative Scheme for QCD: the Four-Gluon Vertex
We study the self-consistency problem of the generalized Feynman rule
(nonperturbatively modified vertex of zeroth perturbative order) for the
4-gluon vertex function in the framework of an extended perturbation scheme
accounting for non-analytic coupling dependence through the Lambda scale.
Tensorial structure is restricted to a minimal dynamically closed basis set.
The self-consistency conditions are obtained at one loop, in Landau gauge, and
at the lowest approximation level (r=1) of interest for QCD. At this level,
they are found to be linear in the nonperturbative 4-gluon coefficients, but
strongly overdetermined due to the lack of manifest Bose symmetry in the
relevant Dyson-Schwinger equation. The observed near decoupling from the
2-and-3-point conditions permits least-squares quasisolutions for given
2-and-3-point input within an effective one-parameter freedom. We present such
solutions for N_F=2 massless quarks and for the pure gluon theory, adapted to
the 2-and-3-point coefficients determined previously.Comment: 46 pages, 11 figure
Extended Iterative Scheme for QCD: Three-point Vertices
In the framework of a generalized iterative scheme introduced previously to
account for the non-analytic coupling dependence associated with the
renormalization-group invariant mass scale Lambda, we establish the
self-consistency equations of the extended Feynman rules (Lambda-modified
vertices of zeroth perturbative order) for the three-gluon vertex, the two
ghost vertices, and the two vertices of massless quarks. Calculations are
performed to one-loop-order, in Landau gauge, and at the lowest approximation
level (r=1) of interest for QCD. We discuss the phenomenon of compensating
poles inherent in these equations, by which the formalism automatically cancels
unphysical poles on internal lines, and the role of composite-operator
information in the form of equation-of-motion condensate conditions. The
observed near decoupling of the four-gluon conditions permits a solution to the
2-and-3-point conditions within an effective one-parameter freedom. There
exists a parameter range in which one solution has all vertex coefficients
real, as required for a physical solution, and a narrower range in which the
transverse-gluon and massless-quark propagators both exhibit complex-conjugate
pole pairs.Comment: 28 pages, 7 figure
Uniaxial stress tuning of geometrical frustration in a Kondo lattice
Hexagonal CeRhSn with paramagnetic moments on a distorted Kagome lattice
displays zero-field quantum critical behavior related to geometrical
frustration. We report high-resolution thermal expansion and magnetostriction
measurements under multiextreme conditions such as uniaxial stress up to 200
MPa, temperatures down to 0.1 K and magnetic fields up to 10 T. Under uniaxial
stress along the -direction, quantum criticality disappears and a complex
magnetic phase diagram arises with a sequence of phases below 1.2 K and fields
between 0 and 3 T (). Since the Kondo coupling increases with
stress, which alone would stabilize paramagnetic behavior in CeRhSn, the
observed order arises from the release of geometrical frustration by in-plane
stress.Comment: Accepted in PRB Rapid Com
Three-Nucleon Force and the -Mechanism for Pion Production and Pion Absorption
The description of the three-nucleon system in terms of nucleon and
degrees of freedom is extended to allow for explicit pion production
(absorption) from single dynamic de-excitation (excitation) processes.
This mechanism yields an energy dependent effective three-body hamiltonean. The
Faddeev equations for the trinucleon bound state are solved with a force model
that has already been tested in the two-nucleon system above pion-production
threshold. The binding energy and other bound state properties are calculated.
The contribution to the effective three-nucleon force arising from the pionic
degrees of freedom is evaluated. The validity of previous coupled-channel
calculations with explicit but stable isobar components in the
wavefunction is studied.Comment: 23 pages in Revtex 3.0, 9 figures (not included, available as
postscript files upon request), CEBAF-TH-93-0
Low-energy electronic properties of clean CaRuO: elusive Landau quasiparticles
We have prepared high-quality epitaxial thin films of CaRuO with residual
resistivity ratios up to 55. Shubnikov-de Haas oscillations in the
magnetoresistance and a temperature dependence in the electrical
resistivity only below 1.5 K, whose coefficient is substantially suppressed in
large magnetic fields, establish CaRuO as a Fermi liquid (FL) with
anomalously low coherence scale. Non-Fermi liquid (NFL) dependence is
found between 2 and 25 K. The high sample quality allows access to the
intrinsic electronic properties via THz spectroscopy. For frequencies below 0.6
THz, the conductivity is Drude-like and can be modeled by FL concepts, while
for higher frequencies non-Drude behavior, inconsistent with FL predictions, is
found. This establishes CaRuO as a prime example of optical NFL behavior in
the THz range.Comment: 12 pages, 21 figures including supplemental materia
Antigen Presentation and Allogeneic Stimulation by Langerhans Cells
Isolated Langerhans cells were studied for 2 immunologic functions, the ability to present antigen to sensitized T lymphocytes and the ability to act as stimulator cells for mixed lymphocyte reactions. Langerhans cells can perform both of these functions. This fact, with the previous finding that Langerhans cells possess surface la antigens and Fe and C3 receptors, strongly suggests that Langerhans cells act as epidermal macrophages
A Systematic Extended Iterative Solution for QCD
An outline is given of an extended perturbative solution of Euclidean QCD
which systematically accounts for a class of nonperturbative effects, while
allowing renormalization by the perturbative counterterms. Proper vertices
Gamma are approximated by a double sequence Gamma[r,p], with r the degree of
rational approximation w.r.t. the QCD mass scale Lambda, nonanalytic in the
coupling g, and p the order of perturbative corrections in g-squared,
calculated from Gamma[r,0] - rather than from the perturbative Feynman rules
Gamma(0)(pert) - as a starting point. The mechanism allowing the
nonperturbative terms to reproduce themselves in the Dyson-Schwinger equations
preserves perturbative renormalizability and is tied to the divergence
structure of the theory. As a result, it restricts the self-consistency problem
for the Gamma[r,0] rigorously - i.e. without decoupling approximations - to the
superficially divergent vertices. An interesting aspect of the scheme is that
rational-function sequences for the propagators allow subsequences describing
short-lived excitations. The method is calculational, in that it allows known
techniques of loop computation to be used while dealing with integrands of
truly nonperturbative content.Comment: 48 pages (figures included). Scope of replacement: correction of a
technical defect; no changes in conten
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