3,598 research outputs found
Roper Electroproduction Amplitudes in a Chiral Confinement Model
A description of the Roper using the chiral chromodielectric model is
presented and the transverse and the scalar helicity
amplitudes for the electromagnetic Nucleon--Roper transition are obtained for
small and moderate . The sign of the amplitudes is correct but the model
predictions underestimate the data at the photon point. Our results do not
indicate a change of sign in any amplitudes up to GeV. The
contribution of the scalar meson excitations to the Roper electroproduction is
taken into account but it turns out to be small in comparison with the quark
contribution. However, it is argued that mesonic excitations may play a more
prominent role in higher excited states.Comment: 10 pages, 8 figures, uses World Scientific macros. Talk presented at
EMI2001 in Osaka, Japa
N^* electroproduction amplitudes in a model with dynamical confinement
The Roper resonance is described in a chiral version of the chromodielectric
model as a cluster of three quarks in radial-orbital configuration
(1s)(2s), surrounded by and -meson clouds and by a
chromodielectric field which assures quark dynamical confinement. Radial
profiles for all fields are determined self-consistently for each baryon.
Transverse and scalar helicity amplitudes for the
nucleon-Roper transition are calculated. The contribution of glueball and
-meson vibrations is estimated; although small for N(1440), the
contribution can be large for N(1710).Comment: 12 pages, 6 figures, uses elsevier macro
Metallic slabs: Perturbative treatments based on jellium
We examine first-order perturbative results based on jellium for the surface
energy of slabs of simple metals, using various local pseudopotentials
(Ashcroft, Heine-Abarenkov and evanescent core). The difference between the
pseudopotential and the jellium potential is averaged along the plane parallel
to the surface. We compare these perturbative results with those of the
stabilized jellium model (a modification of the regular jellium model in which
the perturbation appears in the energy functional right from the outset) and
with the output of other perturbative and non-perturbative calculations.Comment: 15 pages, 2 figures, to appear in Prog. Surf. Sc
Dynamical nature of the nuclear pseudospin and its isospin asymmetry
Pseudospin symmetry in nuclei is investigated by solving the Dirac equation
with Woods-Saxon scalar and vector radial potentials. We relate the pseudospin
interaction with a pseudospin-orbit term in a Schroedinger-like equation for
the lower component of the Dirac spinor. We show that this term gives a large
contribution to the energy splittings of pseudospin partners, so that the near
pseudospin degeneracy arises from a significant cancellation among the
different terms in that equation. This is a manifestation of the dynamical
character of this symmetry in the nucleus. We analyze the isospin dependence of
the pseudospin symmetry in a nuclear isotope chain by including a
vector-isovector potential V_rho and a Coulomb potential and conclude that
V_rho gives the main contribution to the observed pseudospin isospin asymmetry.Comment: 4 pages, 2 figures, uses World Scientific style file. Contribution
presented at the VIII International Workshop on Hadron Physics, Bento
Goncalves, RS, Brazil, April 14-19, 2002. To be published by World Scientific
in the proceedings of the "International Workshop on Hadron Physics 2002
Investigation of the existence of hybrid stars using Nambu-Jona-Lasinio models
We investigate the hadron-quark phase transition inside neutron stars and
obtain mass-radius relations for hybrid stars. The equation of state for the
quark phase using the standard NJL model is too soft leading to an unstable
star and suggesting a modification of the NJL model by introducing a momentum
cutoff dependent on the chemical potential. However, even in this approach, the
instability remains. In order to remedy the instability we suggest the
introduction of a vector coupling in the NJL model, which makes the EoS
stiffer, reducing the instability. We conclude that the possible existence of
quark matter inside the stars require high densities, leading to very compact
stars.Comment: 4 pages, 2 figures; prepared for IV International Workshop on
Astronomy and Relativistic Astrophysics (IWARA 2009), Maresias, 4-8 Oct 200
Small quark stars in the chromodielectric model
Equations of state for strange quark matter in beta equilibrium at high
densities are used to investigate the structure (mass and radius) of compact
objects. The chromodielectric model is used as a general framework for the
quark interactions, which are mediated by chiral mesons, and , and by a confining chiral singlet dynamical field, . Using a
quartic potential for , two equations of state for the same set of model
parameters are obtained, one with a minimum at around the nuclear matter
density and the other at . Using the latter
equation of state in the Tolman-Oppenheimer-Volkoff equations we found
solutions corresponding to compact objects with km and . The phenomenology of recently discovered X-ray sources is compatible
with the type of quark stars that we have obtained.Comment: 8 pages, AIP macros; Talk delivered at the Pan American Advanced
Studies Institute (PASI) Conference "New States of Matter in Hadronic
Interactions", Campos do Jordao, Brazil, January 200
Spin and pseudospin symmetries in the antinucleon spectrum of nuclei
Spin and pseudospin symmetries in the spectra of nucleons and antinucleons
are studied in a relativistic mean-field theory with scalar and vector
Woods-Saxon potentials, in which the strength of the latter is allowed to
change. We observe that, for nucleons and antinucleons, the spin symmetry is of
perturbative nature and it is almost an exact symmetry in the physical region
for antinucleons. The opposite situation is found in the pseudospin symmetry
case, which is better realized for nucleons than for antinucleons, but is of
dynamical nature and cannot be viewed in a perturbative way both for nucleons
and antinucleons. This is shown by computing the spin-orbit and
pseudospin-orbit couplings for selected spin and pseudospin partners in both
spectra.Comment: 8 figures, uses revtex 4.1 macro
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