5,294 research outputs found
The shape of the baryon in a covariant spectator quark model
Using a covariant spectator quark model that describes the recent lattice QCD
data for the electromagnetic form factors and all available
experimental data on transitions, we analyze the charge
and magnetic dipole distributions of the baryon and discuss its shape.
We conclude that the quadrupole moment of the is a good indicator of
the deformation and that the charge distribution has an oblate
shape. We also calculate transverse moments and find that they do not lead to
unambiguous conclusions about the underlying shape.Comment: Extended introduction, references added, other small modifications.
To appear in Phys. Rev. D. 14 pages, 8 figure
Dynamical diversity and metastability in a hindered granular column near jamming
Granular media jam into a panoply of metastable states. The way in which
these states are achieved depends on the nature of local and global constraints
on grains; here we investigate this issue by means of a non-equilibrium
stochastic model of a hindered granular column near its jamming limit. Grains
feel the constraints of grains above and below them differently, depending on
their position. A rich phase diagram with four dynamical phases (ballistic,
activated, logarithmic and glassy) is revealed. The statistics of the jamming
time and of the metastable states reached as attractors of the zero-temperature
dynamics is investigated in each of these phases. Of particular interest is the
glassy phase, where intermittency and a strong deviation from Edwards' flatness
are manifest.Comment: 23 pages, 12 figure
Off-Mass-Shell N Scattering and
We adapt the off-shell N amplitude of the Tucson-Melbourne three-body
force to the half-off-shell amplitude of the pion rescattering contribution to
near threshold. This {\em pion} rescattering contribution,
together with the impulse term, provides a good description of the data when
the half-off-shell amplitude is linked to the phenomenological invariant
amplitudes obtained from meson factory N scattering data.Comment: 3 pages, contributed to STORRI99, Bloomington, Indiana, September
199
Metastable States in High Order Short-Range Spin Glasses
The mean number of metastable states in higher order short-range spin
glasses is estimated analytically using a variational method introduced by
Tanaka and Edwards for very large coordination numbers. For lattices with small
connectivities, numerical simulations do not show any significant dependence on
the relative positions of the interacting spins on the lattice, indicating thus
that these systems can be described by a few macroscopic parameters. As an
extremely anisotropic model we consider the low autocorrelated binary spin
model and we show through numerical simulations that its landscape has an
exceptionally large number of local optima
Covariant spectator theory of np scattering: Phase shifts obtained from precision fits to data below 350 MeV
Using the covariant spectator theory (CST), we present two one boson exchange
kernels that have been successfully adjusted to fit the 2007 world np data
(containing 3788 data) below 350 MeV. One model (which we designate WJC-1) has
27 parameters and fits with a chi2/N = 1.06. The other model (designated WJC-2)
has only 15 parameters and fits with a chi2/N = 1.12. Both of these models also
reproduce the experimental triton binding energy without introducing additional
irreducible three-nucleon forces. One result of this work is a new phase shift
analysis, updated for all data until 2006, which is useful even if one does not
work within the CST. In carrying out these fits we have reviewed the entire
data base, adding new data not previously used in other high precision fits and
restoring some data omitted in previous fits. A full discussion and evaluation
of the 2007 data base is presented.Comment: 43 pages, 27 figures, and 13 table
A covariant constituent-quark formalism for mesons
Using the framework of the Covariant Spectator Theory (CST) [1] we are
developing a covariant model formulated in Minkowski space to study mesonic
structure and spectra. Treating mesons as effective states, we
focused in [2] on the nonrelativistic bound-state problem in momentum space
with a linear confining potential. Although integrable, this kernel has
singularities which are difficult to handle numerically. In [2] we reformulate
it into a form in which all singularities are explicitely removed. The
resulting equations are then easier to solve and yield accurate and stable
solutions. In the present work, the same method is applied to the relativistic
case, improving upon the results of the one-channel spectator equation (1CSE)
given in [3].Comment: 6 pages, 5 figures, Presented at EEF70, Workshop on Unquenched Hadron
Spectroscopy: Non-Perturbative Models and Methods of QCD vs. Experimen
Meson-Meson Interactions and Resonances in the 't Hooft Model
We studied meson-meson interactions using the 't Hooft model, which
represents QCD in 1+1 dimensions and assumes a large number of colors ().
The dominant interactions in this large limit are generated by quark
exchange. Our results show that QCD in 1+1 dimensions allows the realization of
a constituent-type quark model for the mesons, and generates a scalar
``''-like meson-meson resonance, whose effective coupling and mass are
determined by the underlying QCD dynamics. These results suggest an
interpretation of the lightest scalar mesons as systems.Comment: 19 pages, 8 figures, accepted for publication in PR
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