139 research outputs found
Spin observables for pion production from pd collisions
We have calculated the proton analyzing power A_{y0} of the pion-production
reaction from pd collisions for one energy close to threshold and for another
in the region of the -resonance. A fair reproduction of the
experimental data could be obtained in both cases with a model which includes
isoscalar and isovector rescatterings in s waves, as well as the p-wave
rescattering mechanisms mediated by the and vertices.
For the analyzing power at threshold we found that the initial-state
interaction (ISI) is also quite important.Comment: Talk at XVIth International Conference on Few-Body Problems in
Physics, Taipei, Taiwan, March 6-10 2000. 3 pages, latex, 2 figures -- To
appear in Nuclear Physics A -- fleqn.sty and espcrc1.sty require
Three-nucleon portrait with pion
We report on recent results obtained by the above collaboration on the
collision processes involving three nucleons, where we pay particular attention
on the dynamical role of the pion. After discussing the case at intermediate
energies, where real pions can be produced and detected, we have considered the
case at lower energies, where the pions being exchanged are virtual. The study
has revealed the presence of some new pion-exchange mechanisms, which leads to
a new three-nucleon force of tensor structure. Recently, the effect of this
tensor three-nucleon force to the spin observables for neutron-deuteron
scattering at low energy has been analyzed, and will be briefly reviewed.Comment: To appear in the Proceedings of 8th Convegno su Problemi di Fisica
Nucleare Teorica (Cortona 2000), Cortona, Italy, 18-21 Oct 2000. 8 pages,
uses ws-p8-50x6-00.cl
Photonuclear Reactions of Three-Nucleon Systems
We discuss the available data for the differential and the total cross
section for the photodisintegration of He and H and the corresponding
inverse reactions below MeV by comparing with our calculations
using realistic interactions. The theoretical results agree within the
errorbars with the data for the total cross sections. Excellent agreement is
achieved for the angular distribution in case of He, whereas for H a
discrepancy between theory and experiment is found.Comment: 11 pages (twocolumn), 12 postscript figures included, uses psfig,
RevTe
Radiative capture of protons by deuterons
The differential cross section for radiative capture of protons by deuterons
is calculated using different realistic NN interactions. We compare our results
with the available experimental data below . Excellent agreement
is found when taking into account meson exchange currents, dipole and
quadrupole contributions, and the full initial state interaction. There is only
a small difference between the magnitudes of the cross sections for the
different potentials considered. The angular distributions, however, are
practically potential independent.Comment: 4 pages (twocolumn), 4 postscript figures included, submitted for
publication, revised versio
The pd <--> pi+ t reaction around the Delta resonance
The pd pi+ t process has been calculated in the energy region around the
Delta-resonance with elementary production/absorption mechanisms involving one
and two nucleons. The isobar degrees of freedom have been explicitly included
in the two-nucleon mechanism via pi-- and rho-exchange diagrams. No free
parameters have been employed in the analysis since all the parameters have
been fixed in previous studies on the simpler pp pi+ d process. The
treatment of the few-nucleon dynamics entailed a Faddeev-based calculation of
the reaction, with continuum calculations for the initial p-d state and
accurate solutions of the three-nucleon bound-state equation. The integral
cross-section was found to be quite sensitive to the NN interaction employed
while the angular dependence showed less sensitivity. Approximately a 4% effect
was found for the one-body mechanism, for the three-nucleon dynamics in the p-d
channel, and for the inclusion of a large, possibly converged, number of
three-body partial states, indicating that these different aspects are of
comparable importance in the calculation of the spin-averaged observables.Comment: 40 Pages, RevTex, plus 5 PostScript figure
Spin observables for the pd <-> pi+ t process around the Delta resonance
The proton analyzing power Ay0 and the deuteron tensor analyzing power T20
are evaluated for the pd pi+ t process, in the energy region around and
above the Delta resonance. These calculations extend a previous analysis of the
excitation function and differential cross-section, based on a model embodying
one-- and two-body p-wave absorption mechanisms and isobar excitation. The
three-nucleon bound state and the pd scattering state are evaluated through
Faddeev techniques for both the Bonn and Paris potentials. The spin variables
exhibit a greater sensitivity to the number of included three-nucleon partial
waves than the cross-sections, while the role played by the initial-- or
final-state interactions appears to be small. The results for the tensor
analyzing power at backward angles show a non-negligible dependence on the
potentials employed, consistently with what has been previously found for the
cross-sections. The calculation of spin observables gives a clear indication
that other reaction mechanisms (presumably s-wave two-body absorption) have to
be included in the model, in order to reproduce the experimental data below the
Delta-resonance, in analogy with the simpler pp pi+ d process.Comment: 14 pages, REVTeX, plus 6 figs., PostScript (PRC, to be published
Photodisintegration of three- and four- nucleon systems
Three- and four-nucleon photodisintegration processes are quite efficiently
treated by means of effective two-body integral equations in momentum space. We
recall some aspects of their derivation, present previous and most recent
results obtained within this framework, and discuss general features, trends
and effects observed in these investigations: At low energies final-state
interaction plays an important role. Even more pronounced is the effect of
meson exchange currents. A considerable potential dependence shows up in the
low-energy peak region. The different peak heights are found to be closely
correlated with the corresponding binding energies. Above the peak region only
the difference between potentials with or without p-wave contributions remains
relevant. In the differential cross sections the electric quadrupole
contributions have to be taken into account. The remarkable agreement between
theory and experiment in - radiative capture is achieved only when
incorporating this contribution, together with most of the above-mentioned
effects. In the final part of this report we briefly review also methods
developed, and results achieved in three- and four- nucleon
electrodisintegration. We, in particular, compare them with a recent access to
this problem, based on the construction of nucleon-nucleus potentials via
Marchenko inversion theory.Comment: 20 pages LaTeX and 22 postscript figures included, uses epsfig.sty
and espcrc1.sty. Invited talk at the XVth International Conference on
Few-Body Problems in Physics (22-26 July, 1997, Groningen, The Netherlands).
To be published in the conference proceedings in Nucl. Phys.
Nucleon-Nucleon Scattering in a Three Dimensional Approach
The nucleon-nucleon (NN) t-matrix is calculated directly as function of two
vector momenta for different realistic NN potentials. To facilitate this a
formalism is developed for solving the two-nucleon Lippmann-Schwinger equation
in momentum space without employing a partial wave decomposition. The total
spin is treated in a helicity representation. Two different realistic NN
interactions, one defined in momentum space and one in coordinate space, are
presented in a form suited for this formulation. The angular and momentum
dependence of the full amplitude is studied and displayed. A partial wave
decomposition of the full amplitude it carried out to compare the presented
results with the well known phase shifts provided by those interactions.Comment: 26 pages plus 10 jpg figure
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