1,118 research outputs found
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
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.
Constitutional Law - Chilling Effect on First Amendment Rights - Army Surveillance of Civilian Political Activity
The United States Supreme Court has held that allegations of a subjective chill of first amendment rights, due to the mere existence of the Army\u27s intelligence and data gathering system, did not constitute a justiciable controversy since such allegations are not an adequate substitute for a claim of specific present objective harm or a threat of specific future harm necessary to invoke the judicial power to determine the validity of executive or legislative action.
Laird v. Tatum, 408 U.S. 1 (1972)
The scalar glueball spectrum
We discuss scenarios for scalar glueballs using arguments based on sum rules,
spectral decomposition, the approximation, the scales of the
strong interaction and the topology of the flux tubes. We analyze the
phenomenological support of those scenarios and their observational
implications. Our investigations hint a rich low lying glueball spectrum.Comment: 11 pages: New title, figure, table and a more detailed comparison
with experiment
Studying the Sivers function by model calculations
A formalism is presented to evaluate the Sivers function in constituent quark
models. A non-relativistic reduction of the scheme is performed and applied to
the Isgur-Karl model. The sign for the and flavor contributions that we
obtained turns out to be opposite. The Burkardt Sum Rule is fulfilled to a
large extent. After the estimate of the QCD evolution of the results from the
momentum scale of the model to the experimental one, a reasonable agreement
with the available data is obtained.Comment: To appear in the proceedings of Diffraction 2008: International
Workshop On Diffraction In High Energy Physics, 9-14 Sep 2008, La
Londe-les-Maures, France. Misprints corrected, reference adde
Sivers Asymmetry for the proton and the neutron
A formalism is presented to evaluate the Sivers function in constituent quark
models. A non-relativistic reduction of the scheme is performed and applied to
the Isgur-Karl model. 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. The Burkardt Sum Rule is fulfilled to a large extent. After the
estimate of the QCD evolution of the results from the momentum scale of the
model to the experimental one, a reasonable agreement with the available data
is obtained. A calculation of nuclear effects in the extraction of neutron
single spin asymmetries in semi-inclusive deep inelastic scattering off 3He is
also described. In the kinematics of forth-coming experiments at JLab, it is
found that the nuclear effects arising within an Impulse Approximation approach
are under control.Comment: 8 pages, 3 figures, Invited talk at the "6th International Conference
on Hadronic Physics", ICTP, Trieste, Italy, 12-16 May 2008. To appear in AIP
Proceedings Serie
Sivers function in constituent quark models
A formalism to evaluate the Sivers function, developed for calculations in
constituent quark models, is applied to the Isgur-Karl model. A non-vanishing
Sivers asymmetry, with opposite signs for the u and d flavor, is found; the
Burkardt sum rule is fulfilled up to 2 %. Nuclear effects in the extraction of
neutron single spin asymmetries in semi-inclusive deep inelastic scattering off
3He are also evaluated. In the kinematics of JLab, it is found that the nuclear
effects described by an Impulse Approximation approach are under control.Comment: 8 pages, 3 figures, invited talk at the "2nd International Workshop
on Transverse Polarization Phenomena in Hard Processes" (Transversity 2008),
Ferrara, Italy, May 28-31, 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.
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