8,910 research outputs found

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

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

    Nucleon Resonance Effects in ppppπ0pp\to pp\pi^0 near Threshold

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    The role of the low lying nucleon resonances beyond the Δ(1232)\Delta(1232) in the reaction ppppπ0pp\to pp\pi^0 near threshold is shown to be numerically significant by a calculation, which takes into account the pion re-scattering contribution described by chiral perturbation theory and the short-range mechanisms that are implied by the nucleon-nucleon interaction model. The intermediate N(1440) (P11_{11}) resonance is excited by the short-range exchange mechanisms, while the N(1535) (S11S_{11}) and N(1520) (D13D_{13}) resonances are excited by η\eta and ρ\rho meson exchange, respectively. The P11P_{11} increases the calculated cross section, whereas the S11S_{11} and D13D_{13} resonances decrease it. The calculation takes full account of the initial and final state interactions.Comment: Revised accepted versio

    Off-Mass-Shell π\piN Scattering and ppppπ0pp \to pp \pi^0

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    We adapt the off-shell π\piN amplitude of the Tucson-Melbourne three-body force to the half-off-shell amplitude of the pion rescattering contribution to ppppπ0pp \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 π\piN scattering data.Comment: 3 pages, contributed to STORRI99, Bloomington, Indiana, September 199

    Can the Σnn\Sigma^- nn System be Bound?

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    Motivated by the Σ\Sigma-hypernuclear states reported in (K,π±K^-,\pi^{\pm}) experiments, we have explored the possibility that there exists a particle-stable Σnn\Sigma^- nn bound state. For the J\"ulich \~A hyperon-nucleon, realistic-force model, our calculations yield little reason to expect a positive-parity bound state in either the J=12J = \frac{1}{2} or the J=32J = \frac{3}{2} channels.Comment: 6 pages in LaTeX, 1 Figure (appended in uuencoded tar-compressed PostScript format), College of William and Mary preprint WM-94-103, LANL preprint LA-UR-94-023

    Metastable States in High Order Short-Range Spin Glasses

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    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

    A covariant constituent-quark formalism for mesons

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    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 qqˉq\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
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