276 research outputs found
Two-Pion Exchange in Proton-Proton Scattering
The contribution of the box and crossed two-pion-exchange diagrams to
proton-proton scattering at 90 is calculated in the laboratory
momentum range up to 12 GeV/c. Relativistic form factors related to the nucleon
and pion size and representing the pion source distribution based on the quark
structure of the hadronic core are included at each vertex of the pion-nucleon
interaction. These form factors depend on the four-momenta of the exchanged
pions and scattering nucleons. Feynman-diagram amplitudes calculated without
form factors are checked against those derived from dispersion relations. In
this comparison, one notices that a very short-range part of the crossed
diagram, neglected in dispersion-relation calculations of the two-pion-exchange
nucleon-nucleon potential, gives a sizable contribution. In the Feynman-diagram
calculation with form factors the agreement with measured spin-separated cross
sections, as well as amplitudes in the lower part of the energy range
considered, is much better for pion-nucleon pseudo-vector vis \`a vis
pseudo-scalar coupling. While strengths of the box and crossed diagrams are
comparable for laboratory momenta below 2 GeV/c, the crossed diagram dominates
for larger momenta, largely due to the kinematics of the crossed diagram
allowing a smaller momentum transfer in the nucleon center of mass. An
important contribution arises from the principal-value part of the integrals
which is non-zero when form factors are included. It seems that the importance
of the exchange of color singlets may extend higher in energy than expected
Analysis of NN Amplitudes up to 2.5 GeV: An Optical Model and Geometric Interpretation
We analyse the SM97 partial wave amplitudes for nucleon--nucleon (NN)
scattering to 2.5 GeV, in which resonance and meson production effects are
evident for energies above pion production threshold. Our analyses are based
upon boson exchange or quantum inversion potentials with which the
sub-threshold data are fit perfectly. Above 300 MeV they are extrapolations, to
which complex short ranged Gaussian potentials are added in the spirit of the
optical models of nuclear physics and of diffraction models of high energy
physics. The data to 2.5 GeV are all well fit. The energy dependences of these
Gaussians are very smooth save for precise effects caused by the known
and N resonances. With this approach, we confirm that the geometrical
implications of the profile function found from diffraction scattering are
pertinent in the regime 300 MeV to 2.5 GeV and that the overwhelming part of
meson production comes from the QCD sector of the nucleons when they have a
separation of their centres of 1 to 1.2 fm. This analysis shows that the
elastic NN scattering data above 300 MeV can be understood with a local
potential operator as well as has the data below 300 MeV.Comment: 49 pages, including 23 figures, LaTeX2e/RevTeX/ps fil
Nucleon-Nucleon Optical Model for Energies to 3 GeV
Several nucleon-nucleon potentials, Paris, Nijmegen, Argonne, and those
derived by quantum inversion, which describe the NN interaction for T-lab below
300$ MeV are extended in their range of application as NN optical models.
Extensions are made in r-space using complex separable potentials definable
with a wide range of form factor options including those of boundary condition
models. We use the latest phase shift analyses SP00 (FA00, WI00) of Arndt et
al. from 300 MeV to 3 GeV to determine these extensions. The imaginary parts of
the optical model interactions account for loss of flux into direct or resonant
production processes. The optical potential approach is of particular value as
it permits one to visualize fusion, and subsequent fission, of nucleons when
T-lab above 2 GeV. We do so by calculating the scattering wave functions to
specify the energy and radial dependences of flux losses and of probability
distributions. Furthermore, half-off the energy shell t-matrices are presented
as they are readily deduced with this approach. Such t-matrices are required
for studies of few- and many-body nuclear reactions.Comment: Latex, 40 postscript pages including 17 figure
Spin dependence of the antinucleon-nucleon interaction
The status of our present knowledge on the antinucleon-nucleon interaction at
low and medium energies is discussed. Special emphasis is put on aspects
related to its spin dependence which are relevant for experiments planned by
the PAX collaboration. Predictions for the spin-dependent antiproton-proton
cross sections sigma_1 and sigma_2 are presented, utilizing antinucleon-nucleon
potential models developed by the Juelich group, and compared to results based
on the amplitudes of the Nijmegen partial-wave analysis.Comment: 8 pages, 4 figures, to appear in the proceedings of the 19th
International Spin Physics Symposium, September 27 - October 2, 2010,
Juelich, German
Big Bang Nucleosynthesis Constraints on Hadronically and Electromagnetically Decaying Relic Neutral Particles
Big Bang nucleosynthesis in the presence of decaying relic neutral particles
is examined in detail. All non-thermal processes important for the
determination of light-element abundance yields of 2H, 3H, 3He, 4He, 6Li, and
7Li are coupled to the thermonuclear fusion reactions to obtain comparatively
accurate results. Predicted light-element yields are compared to
observationally inferred limits on primordial light-element abundances to infer
constraints on the abundances and properties of relic decaying particles with
decay times in the interval 0.01 sec < tau < 10^(12) sec. Decaying particles
are typically constrained at early times by 4He or 2H, at intermediate times by
6Li, and at large times by the 3He/2H ratio. Constraints are shown for a large
number of hadronic branching ratios and decaying particle masses and may be
applied to constrain the evolution of the early Universe.Comment: 24 pages (revtex), 11 figures, title changed, matches published
versio
Precise method for the determination of the neutron electric form factor based on a relativistic analysis of the process $d(e,e'n)p
We generalize the recoil polarization method for the determination of the
proton form factor to the case of the disintegration of vector polarized
deuterons by longitudinally polarized electrons, . We
suggest to measure for this reaction, in the kinematics of quasi-elastic
-scattering, the ratio of the asymmetries induced by the
- and -components of the deuteron vector polarization. In the framework
of the relativistic impulse approximation the ratio is sensitive to
in a wide interval of momentum transfer squared, whereas it depends
weakly on the details of the -interaction and on the choice of the deuteron
wave function. Moreover, in the range 1.5 GeV, the ratio
shows a smooth dependence on , making the analysis simpler.Comment: 7 pages, 4 figs, 1 tabl
Lambda^0 polarization as a probe for production of deconfined matter in ultra-relativistic heavy-ion collisions
We study the polarization change of Lambda^0's produced in ultra-relativistic
heavy-ion collisions with respect to the polarization observed in proton-proton
collisions as a signal for the formation of a Quark-Gluon Plasma (QGP).
Assuming that, when the density of participants in the collision is larger than
the critical density for QGP formation, the Lambda^0 production mechanism
changes from recombination type processes to the coalescence of free valence
quarks, we find that the Lambda^0 polarization depends on the relative
contribution of each process to the total number of Lambda^0's produced in the
collision. To describe the polarization of Lambda^0's in nuclear collisions for
densities below the critical density for the QGP formation, we use the
DeGrand-Miettinen model corrected for the effects introduced by multiple
scattering of the produced Lambda^0 within the nuclear environment.Comment: 9 pages, 6 figures, uses ReVTeX and epsfig.st
The p(d,p)d and p(d,p)pn reactions as a tool for the study of the short range internal structure of the deuteron
In recent time the deuteron structure at short distances is often treated
from the point of view nonnucleonic degrees of freedom. In this paper the
measurements of T-odd polarization observables using tensor polarized deuteron
beam and polarized proton target or proton polarimeter are proposed to search
the quark configurations inside the deuteron.Comment: 12 pages, 8 Postscript figures, submitted in Phys.Atom.Nuc
Constraints on Parity-Even Time Reversal Violation in the Nucleon-Nucleon System and Its Connection to Charge Symmetry Breaking
Parity-even time reversal violation (TRV) in the nucleon-nucleon interaction
is reconsidered. The TRV -exchange interaction on which recent analyses
of measurements are based is necessarily also charge-symmetry breaking (CSB).
Limits on its strength relative to regular -exchange are
extracted from recent CSB experiments in neutron-proton scattering. The result
(95% CL) is considerably lower than limits
inferred from direct TRV tests in nuclear processes. Properties of
-exchange and limit imposed by the neutron EDM are briefly discussed.Comment: RevTex, 8 pages. Factor ten error in cited neutron EDM corrected,
discussion and two references adde
Dilepton production in proton-proton collisions at BEVALAC energies
The dilepton production in elementary reactions at
BEVALAC energies GeV is investigated. The calculations
include direct decays of the vector mesons , , and , Dalitz decays of the -, -, -, -, and -mesons, and of the baryon resonances
. The subthreshold vector meson production cross sections in
collisions are treated in a way sufficient to avoid double counting with the
inclusive vector meson production. The vector meson dominance model for the
transition form factors of the resonance Dalitz decays is
used in an extended form to ensure correct asymptotics which are in agreement
with the quark counting rules. Such a modification gives an unified and
consistent description of both radiative decays and meson decays.
The effect of multiple pion production on the experimental efficiency for the
detection of the dilepton pairs is studied. We find the dilepton yield in
reasonable agreement with the experimental data for the set of intermediate
energies whereas at the highest energy GeV the number of
dilepton pairs is likely to be overestimated experimentally in the mass range
MeV.Comment: 25 pages (IOP style), 5 figures, revised manuscript accepted for
publication in JP
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