978 research outputs found
(e,e'N) and (e,e'NN) experiments at NIKHEF, Mainz and JLab and open problems in one- and two-nucleon emission
A critical review is presented of recent data on direct one- and two-nucleon
emission obtained in electron scattering at NIKHEF, Mainz and JLab. In the case
of (e,ep) reactions attention is focussed on extracting spectroscopic
factors, looking at medium effects on the bound nucleon form factors, and
investigating nuclear transparency. The possibility of obtaining information on
nucleon-nucleon correlations in (e,epp) and (e,epn) reactions is also
discussed.Comment: 14 pages, LaTeX, four eps figures, style file included. Invited talk
at the International Workshop on Nuclear Many-Body Problem and Sub-Nucleonic
Degrees of Freedom in Nuclei, Jilin University, Changchun, China, July 2-8,
200
Nuclear Compton scattering in the -resonance region with polarized photons
Nuclear Compton scattering in the -resonance region is reconsidered
within the framework of the -hole model. The different role of the
resonant and non-resonant contributions to the transition amplitudes is
discussed and their effect is investigated by comparing the results of
calculation with recent data also taken with polarized photons.Comment: 22 pages, LaTeX, 7 figures, postscript files available from the
Author
Electroweak structure of the nucleon, meson cloud and light-cone wavefunctions
The meson-cloud model of the nucleon consisting of a system of three valence
quarks surrounded by a meson cloud is applied to study the electroweak
structure of the proton and neutron. Light-cone wavefunctions are derived for
the dressed nucleon as pictured to be part of the time a bare nucleon and part
of the time a baryon-meson system. Configurations are considered where the
baryon can be a nucleon or a \Delta and the meson can be a pion as well as a
vector meson such as the \rho or the \omega. An overall good description of the
electroweak form factors is obtained. The contribution of the meson cloud is
small and only significant at low Q^2. Mixed-symmetry S'-wave components in the
wavefunction are most important to reproduce the neutron electric form factor.
Charge and magnetization densities are deduced as a function of both the radial
distance from the nucleon center and the transverse distance with respect to
the direction of the three-momentum transfer. In the latter case a central
negative charge is found for the neutron. The up and down quark distributions
associated with the Fourier transform of the axial form factor have opposite
sign, with the consequence that the probability to find an up (down) quark with
positive helicity is maximal when it is (anti)aligned with the proton helicity.Comment: references updated and typos in figure 2 corrected; to be published
in Phys. Rev.
Generalized parton distributions in a meson cloud model
We present a model calculation of the generalized parton distributions where
the nucleon is described by a quark core surrounded by a mesonic cloud. In the
one-meson approximation, we expand the Fock state of the physical nucleon in a
series involving a bare nucleon and two-particle, meson-baryon, states. We
discuss the role of the different Fock-state components of the nucleon by
deriving a convolution formalism for the unpolarized generalized parton
distributions, and showing predictions at different kinematics.Comment: 7 pages, 3 figures; Invited talk at the Fifth International
Conference on Perspectives in Hadronic Physics, Miramare - Trieste (Italy),
22-26 May 200
Wigner distributions and quark orbital angular momentum
We discuss the quark phase-space or Wigner distributions of the nucleon which
combine in a single picture all the information contained in the generalized
parton distributions and the transverse-momentum dependent parton
distributions. In particular, we present results for the distribution of
unpolarized quarks in a longitudinally polarized nucleon obtained in a
light-front constituent quark model. We show how the quark orbital angular
momentum can be extracted from the Wigner distributions and compare it with
alternative definitions.Comment: 8 pages, 2 figures, 1 table, contribution to the proceedings of the
QCD Evolution workshop 2012, May 14-17, JLa
Chiral-odd generalized parton distributions in constituent quark models
We derive the overlap representation of chiral-odd generalized parton
distributions using the Fock-state decomposition in the transverse-spin basis.
This formalism is applied to the case of light-cone wave functions in a
constituent quark model. Numerical results for the four chiral-odd generalized
parton distributions at the hadronic scale are shown in different kinematics.
In the forward limit we derive the transversity distribution, the tensor charge
and the angular momentum sum rule for quarks with transverse polarization in an
unpolarized nucleon.Comment: 23 pages, 7 figures; one reference added and one formula corrected;
conclusions and discussions unchange
Chiral-Odd Generalized Parton Distributions, Transversity and Double Transverse-Spin Asymmetry in Drell-Yan Dilepton Production
Within the framework of light-cone quantization we derive the overlap
representation of generalized parton distributions for transversely polarized
quarks using the Fock-state decomposition in the transverse-spin basis. We
apply this formalism to the case of light-cone wave functions in a constituent
quark model giving numerical results for the four chiral-odd generalized parton
distributions in a region where they describe the emission and reabsorption of
a quark by the nucleon. With the transversity distribution obtained in the
forward limit of the generalized distribution, we provide some predictions for
the double transverse-spin asymmetry in Drell-Yan dilepton production in the
kinematics of the PAX experiment.Comment: 8 pages, 4 figures. To appear in the Proceedings of the "XI
Conference on Problems in Theoretical Nuclear Physics", Cortona, Italy, 11-14
October 200
Chiral-Odd Generalized Parton Distributions and Transversity in Light-Front Constituent Quark Models
We present the general framework to calculate chiral-odd generalized parton
distributions in the overlap representation using the Fock-state decomposition
in the transverse spin basis. In the forward limit we derive the transversity
distribution, the tensor charge and the angular momentum sum rule for quarks
with transverse polarization in an unpolarized nucleon. Numerical results are
obtained by applying the formalism to the case of light-cone wavefunctions of
constituent quark models.Comment: 8 pages, 3 figures. Invited talk at the "Advanced Studies Institute
on SYMMETRIES AND SPIN" (SPIN-Praha-2006), 19-26 July, 2006, Prague. To be
pubblished in Czechoslovak Journal of Physics. Some references adde
The spin structure of the nucleon in light-cone quark models
The quark spin densities related to generalized parton distributions in
impact-parameter space and to transverse-momentum dependent parton
distributions are reviewed within a light-cone quark model, with focus on the
role of the different spin-spin and spin-orbit correlations of quarks.Results
for azimuthal spin asymmetries in semi-inclusive deep-inelastic scattering due
to T-even transverse-momentum dependent parton distributions are also
discussed.Comment: proceedings of the workshop "Recent Advances in Perturbative QCD and
Hadronic Physics", 20-25 July 2009, ECT*, Trento (Italy), in Honor of Prof.
Anatoly Efremov's 75th Birthday Celebration; also based on the talk given at
the workshop "Transverse Partonic Structure of Hadrons", 21-26 June 2009,
Yerevan (Armenia); to appear in Mod. Phys. Lett.
Semi-inclusive structure functions in the spectator model
We establish the relationship between distribution and fragmentation
functions and the structure functions appearing in the cross section of
polarized 1-particle inclusive deep-inelastic scattering. We present spectator
model evaluations of these structure functions focusing on the case of an
outgoing spin-1/2 baryon. Distribution functions obtained in the spectator
model are known to fairly agree at low energy scales with global
parameterizations extracted, for instance, from totally inclusive DIS data.
Therefore, we expect it to give good hints on the functional dependence of the
structure functions on the scaling variables x(Bjorken), z and on the
transverse momentum of the observed outgoing hadron, P_{h\perp}. Presently,
this dependence is not very well known, but experiments are planned in the near
future.Comment: 19 pages, 16 figures, submitted to Eur. Phys. J.
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