291 research outputs found
Pions in the nuclear medium and Drell-Yan scattering
We investigate the modification of the pion-cloud in the nuclear medium and
its effect on the nuclear Drell-Yan process. The pion's in-medium self-energy
is calculated in a self-consistent delta-hole model, with particle-hole
contribution also included. Both the imaginary and real part of the pion's and
delta's self-energy are taken into account and related through a dispersion
relation assuring causality. The resulting in-medium pion light-cone momentum
distribution shows only a slight enhancement compared to the one of the free
nucleon. As a consequence the ratio of the cross-section for Drell-Yan
scattering on nuclear matter and nucleonic target is close to unity in
agreement with experiment.Comment: 33 pages, Latex with epsf, figures included, to appear in Phys. Rev.
Nuclear Shadowing in a Parton Recombination Model
Deep inelastic structure functions are investigated in a
rescaling model with parton recombination effects. We find that the model can
explain experimentally measured structure functions reasonably well
in the wide Bjorken range (). In the very small region
(), recombination results are very sensitive to input sea-quark and
gluon distributions.Comment: preprint MKPH-T-93-04, IU/NTC 92-20, 25 pages, TEX file (without
Figs. 1-14)., (address after April 1: Saga U., Japan
THE INTERPLAY OF THE K+K- ATOM AND THE f_0(975) RESONANCE
We predict that production of the K+K- atom in pd-3^HeX and similar reactions
exhibits a drastic missing mass spectrum due to the interplay with f_0(975)
resonance. We point out that high precision studies of the K+K- atom may shed a
new light on the nature of f_0(975).Comment: 13 page
Return of the EMC Effect: Finite Nuclei
A light front formalism for deep inelastic lepton scattering from finite
nuclei is developed. In particular, the nucleon plus momentum distribution and
a finite system analog of the Hugenholtz-van Hove theorem are presented. Using
a relativistic mean field model, numerical results for the plus momentum
distribution and ratio of bound to free nucleon structure functions for Oxygen,
Calcium and Lead are given. We show that we can incorporate light front physics
with excellent accuracy while using easily computed equal time wavefunctions.
Assuming nucleon structure is not modified in-medium we find that the
calculations are not consistent with the binding effect apparent in the data
not only in the magnitude of the effect, but in the dependence on the number of
nucleons.Comment: 11 pages, 6 figure
A Light Front Treatment of the Nucleus-Implications for Deep Inelastic Scattering
A light front treatment of the nuclear wave function is developed and
applied, using the mean field approximation, to infinite nuclear matter. The
nuclear mesons are shown to carry about a third of the nuclear plus momentum,
p+; but their momentum distribution has support only at p+ =0, and the mesons
do not contribute to nuclear deep inelastic scattering. This zero mode effect
occurs because the meson fields are independent of space-time position.Comment: 11 pages, revtex, 1 figur
Pion Excess, Nuclear Correlations, and the Interpretation of () Spin Transfer Experiments
Conventional theories of nuclear interactions predict a net increase in the
distribution of virtual pions in nuclei relative to free nucleons. Analysis of
data from several nuclear experiments has led to claims of evidence against
such a pion excess. These conclusions are usually based on a collective theory
(RPA) of the pions, which may be inadequate. The issue is the energy dependence
of the nuclear response, which differs for theories with strong NN correlations
from the RPA predictions. In the present paper, information about the energy
dependence is extracted from sum rules, which are calculated for such a
correlated, noncollective nuclear theory. The results lead to much reduced
sensitivity of nuclear reactions to the correlations that are responsible for
the pion excess. The primary example is spin transfer, for
which the expected effects are found to be smaller than the experimental
uncertainties. The analysis has consequences for Deep Inelastic Scattering
(DIS) experiments as well.Comment: 16 pages, LaTeX, no figures, submitted to Phys. Rev.
Medium Modifications of Hadron Properties and Partonic Processes
Chiral symmetry is one of the most fundamental symmetries in QCD. It is
closely connected to hadron properties in the nuclear medium via the reduction
of the quark condensate , manifesting the partial restoration of
chiral symmetry. To better understand this important issue, a number of
Jefferson Lab experiments over the past decade have focused on understanding
properties of mesons and nucleons in the nuclear medium, often benefiting from
the high polarization and luminosity of the CEBAF accelerator. In particular, a
novel, accurate, polarization transfer measurement technique revealed for the
first time a strong indication that the bound proton electromagnetic form
factors in 4He may be modified compared to those in the vacuum. Second, the
photoproduction of vector mesons on various nuclei has been measured via their
decay to e+e- to study possible in-medium effects on the properties of the rho
meson. In this experiment, no significant mass shift and some broadening
consistent with expected collisional broadening for the rho meson has been
observed, providing tight constraints on model calculations. Finally, processes
involving in-medium parton propagation have been studied. The medium
modifications of the quark fragmentation functions have been extracted with
much higher statistical accuracy than previously possible.Comment: to appear in J. Phys.: Conf. Proc. "New Insights into the Structure
of Matter: The First Decade of Science at Jefferson Lab", eds. D.
Higinbotham, W. Melnitchouk, A. Thomas; added reference
Developing a framework for the analysis of power through depotentia
Stakeholder participation in tourism policy-making is usually perceived as providing a means of empowerment. However participatory processes drawing upon stakeholders from traditionally empowered backgrounds may provide the means of removing empowerment from stakeholders. Such an outcome would be in contradiction to the claims that participatory processes improve both inclusivity and sustainability. In order to form an understanding of the sources through which empowerment may be removed, an analytical perspective has been developed deriving from Lukes�s views of power dating from 1974. This perspective considers the concept of depotentia as the removal of �power to� without speculating upon the underlying intent and also provides for the multidimensionality of power to be examined within a single study. The application of this analytical perspective has been tested upon findings of the government-commissioned report of the Countryside and Community Research Unit in 2005. The survey and report investigated the progress of Local Access Forums in England created in response to the Countryside and Rights of Way Act 2000. Consideration of the data from this perspective permits the classification of individual sources of depotentia which can each be addressed and potentially enable stakeholder groups to reverse loss of empowerment where it has occurred
Flavor and Charge Symmetry in the Parton Distributions of the Nucleon
Recent calculations of charge symmetry violation(CSV) in the valence quark
distributions of the nucleon have revealed that the dominant symmetry breaking
contribution comes from the mass associated with the spectator quark
system.Assuming that the change in the spectator mass can be treated
perturbatively, we derive a model independent expression for the shift in the
parton distributions of the nucleon. This result is used to derive a relation
between the charge and flavor asymmetric contributions to the valence quark
distributions in the proton, and to calculate CSV contributions to the nucleon
sea. The CSV contribution to the Gottfried sum rule is also estimated, and
found to be small
Light-Front Nuclear Physics: Mean Field Theory for Finite Nuclei
A light-front treatment for finite nuclei is developed from a relativistic
effective Lagrangian (QHD1) involving nucleons, scalar mesons and vector
mesons. We show that the necessary variational principle is a constrained one
which fixes the expectation value of the total momentum operator to be
the same as that for . This is the same as minimizing the sum of the total
momentum operators: . We obtain a new light-front version of the
equation that defines the single nucleon modes. The solutions of this equation
are approximately a non-trivial phase factor times certain solutions of the
usual equal-time Dirac equation. The ground state wave function is treated as a
meson-nucleon Fock state, and the meson fields are treated as expectation
values of field operators in that ground state. The resulting equations for
these expectation values are shown to be closely related to the usual meson
field equations. A new numerical technique to solve the self-consistent field
equations is introduced and applied to O and Ca. The computed
binding energies are essentially the same as for the usual equal-time theory.
The nucleon plus momentum distribution (probability for a nucleon to have a
given value of ) is obtained, and peaks for values of about seventy
percent of the nucleon mass. The mesonic component of the ground state wave
function is used to determine the scalar and vector meson momentum distribution
functions, with a result that the vector mesons carry about thirty percent of
the nuclear plus-momentum. The vector meson momentum distribution becomes more
concentrated at as increases.Comment: 36 pages, 2 figure
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