135 research outputs found
3He structure from coherent hard exclusive processes
Hard exclusive processes, such as deep electroproduction of photons and
mesons off nuclear targets, could give access, in the coherent channel, to
nuclear generalized parton distributions (GPDs). Here, a realistic microscopic
calculation of the unpolarized quark GPD H_q of the 3He nucleus is reviewed. In
Impulse Approximation, H_q is obtained as a convolution between the GPD of the
internal nucleon and the non-diagonal spectral function, describing properly
Fermi motion and binding effects. The obtained formula has the correct limits.
Nuclear effects, evaluated by a modern realistic potential, are found to be
larger than in the forward case. In particular, they increase with increasing
the momentum transfer and the asymmetry of the process. Another feature of the
obtained results is that the nuclear GPD cannot be factorized into a
Delta^2-dependent and a Delta^2-independent term, as suggested in prescriptions
proposed for finite nuclei. The dependence of the obtained GPDs on different
realistic potentials used in the calculation shows that these quantities are
sensitive to the details of nuclear structure at short distances.Comment: 8 pages, talk given at the "X Convegno su problemi di fisica nucleare
teorica", October 6-9, 2004, Cortona, Italy. To appear in the Proceeding
The effects of nuclear structure on generalized parton distributions of 3He
The effect of the nuclear medium on generalized parton distributions (GPDs)
is studied for the 3He nucleus, through a realistic microscopic analysis. In
Impulse Approximation, Fermi motion and binding effects, evaluated by modern
potentials, are found to be larger than in the forward case and very sensitive
to the details of nuclear structure at short distances.Comment: 4 pages, talk given at the Conference "Baryons 2004", October 25-29
2004, Pailaseau (France), to be published by Nuc. Phys.
Generalized parton distributions and constituent quarks
An approach is described to calculate Generalized Parton Distributions (GPDs)
in Constituent Quark Models (CQM). The GPDs are obtained from wave functions to
be evaluated in a given CQM. The general relations linking the twist-two GPDs
to the form factors and to the leading twist quark densities are recovered.
Results for the leading twist, unpolarized GPD in the Isgur and Karl model are
presented.Comment: 4 pages, 4 eps figures; Contribution to the European Workshop on the
QCD Structure of the Nucleon (QCD-N'02), Ferrara, Italy, 3-6 Apr 200
Parton distributions in a constituent quark scenario
A simple picture of the constituent quark as a composite system of point-like
partons is used to construct the unpolarized and polarized parton distributions
by a convolution between constituent quark momentum distributions and
constituent quark structure functions. We achieve good agreement with
experiments in the unpolarized, as well as, in the polarized case. When our
results are compared with similar calculations using non-composite constituent
quarks, the accord with the experiments of the present scheme is impressive. We
conclude that DIS data are consistent with a low energy scenario dominated by
composite constituents of the nucleon.Comment: 4 pages; latex using espcrc1.sty; 4 postscript figures; Invited talk
at the Workshop ``Nucleon '99'', Frascati; Italy 7-9 June 1999. Submitted to
Nuc. Phys.
Neutron unpolarized structure function F_2^n(x) from deep inelastic scattering off ^{3}He and ^{3}H
The possibility to safely extract the neutron deep inelastic structure
function in the range from joint measurements of
deep inelastic structure functions of and is demonstrated.
While the nuclear structure effects are relevant, the model dependence in this
extraction linked to the interaction is shown to be weak.Comment: 5 pages. Proc. XVIIth Conf. on "Few-Body Problems in Physics", Evora(
Portugal) Sept. 11, 2000. To appear in Nucl. Phys.
Model calculations of the Sivers function satisfying the Burkardt Sum Rule
It is shown that, at variance with previous analyses, the MIT bag model can
explain the available data of the Sivers function and satisfies the Burkardt
Sum Rule to a few percent accuracy. The agreement is similar to the one
recently found in the constituent quark model. Therefore, these two model
calculations of the Sivers function are in agreement with the present
experimental and theoretical wisdom.Comment: 9 pages, 4 figures; one figure added; references added; slightly
revised version accepted for publication in Phys. Rev.
A quark model analysis of the Sivers function
We develop a formalism to evaluate the Sivers function. The approach is well
suited for calculations which use constituent quark models to describe the
structure of the nucleon. A non-relativistic reduction of the scheme is
performed and applied to the Isgur-Karl model of hadron structure. The results
obtained are consistent with a sizable Sivers effect and the signs for the u
and d flavor contributions turn out to be opposite. This pattern is in
agreement with the one found analyzing, in the same model, the impact parameter
dependent generalized parton distributions. The Burkardt Sum Rule turns out to
be fulfilled to a large extent. We estimate the QCD evolution of our results
from the momentum scale of the model to the experimental one and obtain
reasonable agreement with the available data.Comment: 31 pages, 4 figures. Extended version, discussion of the Burkardt Sum
Rule added, references added, minor changes in the numerical results, same
interpretation. Final version accepted for publication in Phys. Rev.
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