The shape of the Δ\Delta baryon in a covariant spectator quark model

Using a covariant spectator quark model that describes the recent lattice QCD data for the $\Delta$ electromagnetic form factors and all available experimental data on $\gamma N \to \Delta$ transitions, we analyze the charge and magnetic dipole distributions of the $\Delta$ baryon and discuss its shape. We conclude that the quadrupole moment of the $\Delta$ is a good indicator of the deformation and that the $\Delta^+$ 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 π\piN Scattering and pp→ppπ0pp \to pp \pi^0

We adapt the off-shell $\pi$N amplitude of the Tucson-Melbourne three-body force to the half-off-shell amplitude of the pion rescattering contribution to $pp \to pp \pi^0$ 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 $\pi$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 $q\bar{q}$ 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 ($N_c$). The dominant interactions in this large $N_c$ 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 ``$\sigma$''-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 $q\bar{q} q \bar{q}$ systems.Comment: 19 pages, 8 figures, accepted for publication in PR