3,232 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
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
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
The MINI mixed finite element for the Stokes problem: An experimental investigation
Super-convergence of order 1.5 in pressure and velocity has been
experimentally investigated for the two-dimensional Stokes problem discretised
with the MINI mixed finite element. Even though the classic mixed finite
element theory for the MINI element guarantees linear convergence for the total
error, recent theoretical results indicate that super-convergence of order 1.5
in pressure and of the linear part of the computed velocity to the piecewise
linear nodal interpolation of the exact velocity is in fact possible with
structured, three-directional triangular meshes. The numerical experiments
presented here suggest a more general validity of super-convergence of order
1.5, possibly to automatically generated and unstructured triangulations. In
addition, the approximating properties of the complete computed velocity have
been compared with the approximating properties of the piecewise-linear part of
the computed velocity, finding that the former is generally closer to the exact
velocity, whereas the latter conserves mass better
Border bases for lattice ideals
The main ingredient to construct an O-border basis of an ideal I
K[x1,. .., xn] is the order ideal O, which is a basis of the K-vector space
K[x1,. .., xn]/I. In this paper we give a procedure to find all the possible
order ideals associated with a lattice ideal IM (where M is a lattice of Z n).
The construction can be applied to ideals of any dimension (not only
zero-dimensional) and shows that the possible order ideals are always in a
finite number. For lattice ideals of positive dimension we also show that,
although a border basis is infinite, it can be defined in finite terms.
Furthermore we give an example which proves that not all border bases of a
lattice ideal come from Gr\"obner bases. Finally, we give a complete and
explicit description of all the border bases for ideals IM in case M is a
2-dimensional lattice contained in Z 2 .Comment: 25 pages, 3 figures. Comments welcome!, MEGA'2015 (Special Issue),
Jun 2015, Trento, Ital
Inclusive quasielastic scattering of polarized electrons from polarized nuclei
The inclusive quasielastic response functions that appear in the scattering
of polarized electrons from polarized nuclei are computed and analyzed for
several closed-shell-minus-one nuclei with special attention paid to 39K.
Results are presented using two models for the ejected nucleon --- when
described by a distorted wave in the continuum shell model or by a plane wave
in PWIA with on- and off-shell nucleons. Relativistic effects in kinematics and
in the electromagnetic current have been incorporated throughout. Specifically,
the recently obtained expansion of the electromagnetic current in powers only
of the struck nucleon's momentum is employed for the on-shell current and the
effects of the first-order terms (spin-orbit and convection) are compared with
the zeroth-order (charge and magnetization) contributions. The use of polarized
inclusive quasielastic electron scattering as a tool for determining
near-valence nucleon momentum distributions is discussed.Comment: 51 LaTeX pages, 14 Postscript figure
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 of the nucleon in constituent quark models
Generalized parton distributions (GPDs) are studied at the hadronic
(nonperturbative) scale within different assumptions based on a relativistic
constituent quark model. In particular, by means of a meson-cloud model we
investigate the role of nonperturbative antiquark degrees of freedom and the
valence quark contribution. A QCD evolution of the obtained GPDs is used to add
perturbative effects and to investigate the GPDs' sensitivity to the
nonperturbative ingredients of the calculation at larger (experimental) scale.Comment: 4 pages, 1 figure, presented at the Baryon-04 Conference, Palaiseau
(France), October 25-29, 2004. Nuclear Physics A, to be publishe
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
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