30,175 research outputs found
Two-pion exchange potential and the amplitude
We discuss the two-pion exchange potential which emerges from a box diagram
with one nucleon (the spectator) restricted to its mass shell, and the other
nucleon line replaced by a subtracted, covariant scattering amplitude
which includes , Roper, and isobars, as well as contact terms
and off-shell (non-pole) dressed nucleon terms. The amplitude satisfies
chiral symmetry constraints and fits data below 700 MeV pion
energy. We find that this TPE potential can be well approximated by the
exchange of an effective sigma and delta meson, with parameters close to the
ones used in one-boson-exchange models that fit data below the pion
production threshold.Comment: 9 pages (RevTex) and 7 postscript figures, in one uuencoded gzipped
tar fil
High-precision covariant one-boson-exchange potentials for np scattering below 350 MeV
All realistic potential models for the two-nucleon interaction are to some
extent based on boson exchange. However, in order to achieve an essentially
perfect fit to the scattering data, characterized by a chi2/Ndata ~ 1, previous
potentials have abandoned a pure one boson-exchange mechanism (OBE). Using a
covariant theory, we have found a OBE potential that fits the 2006 world np
data below 350 MeV with a chi2/Ndata = 1.06 for 3788 data. Our potential has
fewer adjustable parameters than previous high-precision potentials, and also
reproduces the experimental triton binding energy without introducing
additional irreducible three-nucleon forces.Comment: 4 pages; revised version with augmented data sets; agrees with
published versio
A comprehensive treatment of electromagnetic interactions and the three-body spectator equations
We present a general derivation the three-body spectator (Gross) equations
and the corresponding electromagnetic currents. As in previous paper on
two-body systems, the wave equations and currents are derived from those for
Bethe-Salpeter equation with the help of algebraic method using a concise
matrix notation. The three-body interactions and currents introduced by the
transition to the spectator approach are isolated and the matrix elements of
the e.m. current are presented in detail for system of three indistinguishable
particles, namely for elastic scattering and for two and three body break-up.
The general expressions are reduced to the one-boson-exchange approximation to
make contact with previous work. The method is general in that it does not rely
on introduction of the electromagnetic interaction with the help of the minimal
replacement. It would therefore work also for other external fields
Normalization of the covariant three-body bound state vertex function
The normalization condition for the relativistic three nucleon Bethe-Salpeter
and Gross bound state vertex functions is derived, for the first time, directly
from the three body wave equations. It is also shown that the relativistic
normalization condition for the two body Gross bound state vertex function is
identical to the requirement that the bound state charge be conserved, proving
that charge is automatically conserved by this equation.Comment: 24 pages, 9 figures, published version, minor typos correcte
The stability of the spectator, Dirac, and Salpeter equations for mesons
Mesons are made of quark-antiquark pairs held together by the strong force.
The one channel spectator, Dirac, and Salpeter equations can each be used to
model this pairing. We look at cases where the relativistic kernel of these
equations corresponds to a time-like vector exchange, a scalar exchange, or a
linear combination of the two. Since the model used in this paper describes
mesons which cannot decay physically, the equations must describe stable
states. We find that this requirement is not always satisfied, and give a
complete discussion of the conditions under which the various equations give
unphysical, unstable solutions
Quark-Antiquark Bound States in the Relativistic Spectator Formalism
The quark-antiquark bound states are discussed using the relativistic
spectator (Gross) equations. A relativistic covariant framework for analyzing
confined bound states is developed. The relativistic linear potential developed
in an earlier work is proven to give vanishing meson decay
amplitudes, as required by confinement. The regularization of the singularities
in the linear potential that are associated with nonzero energy transfers (i.e.
) is improved. Quark mass functions that build chiral
symmetry into the theory and explain the connection between the current quark
and constituent quark masses are introduced. The formalism is applied to the
description of pions and kaons with reasonable results.Comment: 31 pages, 16 figure
Spin and angular momentum in the nucleon
Using the covariant spectator theory (CST), we present the results of a
valence quark-diquark model calculation of the nucleon structure function f(x)
measured in unpolarized deep inelastic scattering (DIS), and the structure
functions g1(x) and g2(x) measured in DIS using polarized beams and targets.
Parameters of the wave functions are adjusted to fit all the data. The fit
fixes both the shape of the wave functions and the relative strength of each
component. Two solutions are found that fit f(x) and g1(x), but only one of
these gives a good description of g2(x). This fit requires the nucleon CST wave
functions contain a large D-wave component (about 35%) and a small P-wave
component (about 0.6%). The significance of these results is discussed.Comment: 27 pages; 13 figure
Covariant spectator theory for the electromagnetic three-nucleon form factors: Complete impulse approximation
We present the first calculations of the electromagnetic form factors of
He and H within the framework of the Covariant Spectator Theory (CST).
This first exploratory study concentrates on the sensitivity of the form
factors to the strength of the scalar meson-nucleon off-shell coupling, known
from previous studies to have a strong influence on the three-body binding
energy. Results presented here were obtained using the complete impulse
approximation (CIA), which includes contributions of relativistic origin that
appear as two-body corrections in a non-relativistic framework, such as
"Z-graphs", but omits other two and three-body currents. We compare our results
to non-relativistic calculations augmented by relativistic corrections of
.Comment: 30 pages, 13 figures, 6 table
Discrimination of Coastal Vegetation and Biomass Using AIS Data
The Airborne Imaging Spectrometer (AIS) was flown over a coastal wetlands region near Lewes, Delaware, adjacent to the Delaware Bay on 16 August 1984. Using the AIS data, it was possible to discriminate between four different types of wetland vegetation canopies: (1) trees; (2) broadleaf herbaceous plants (e.g., Acnida cannabina, Hisbiscus moscheutos); (3) the low marsh grass Spartina alterniflora; and (4) the high marsh grasses Distichlis spicata and Spartina patens. The single most useful region of the spectrum was that between 1.40 and 1.90 microns, where slopes of portions of the radiance curve and ratios of radiance at particular wavelengths were significantly different for the four canopy types. The ratio between the highest digital number in the 1.40 to 1.90 microns and .84 to .94 microns regions and a similar ratio between the peaks in radiance in the 1.12 to 1.40 microns and .84 to .94 microns spectral regions were also very effective at discriminating between vegetation types. Differences in radiance values at various wavelengths between samples of the same vegetation type could potentially be used to estimate biomass
Electromagnetic structure of the deuteron: review of recent theoretical and experimental results
This talk reviews recent theoretical and experimental results for elastic
electron deuteron scattering (yielding the deuteron form factors), threshold
electrodisintegration ( where the mass of the final pair,
, is only a few MeV above the threshold value of ), and high energy
deuteron photodisintegration (). The talk is based on the
complete reviews of Refs. [GVO,S,GG], with a few new results not previously
reported.Comment: 6 pages, 10 figures, Invited talk for Workshop on Electron Nucleus
Scattering VII, Elba, Italy, 200
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