Vector Mesons and Baryon Resonances in Nuclear Matter

We calculate the effect of many-body interactions in nuclear matter on the spectral function of $\rho$ and $\omega$ meson. In particular, we focus on the role played by baryon resonances in this context.Comment: 4 pages, 4 figures, to be published in proceedings of the Third International Conference on Perspectives on Hadronic Physics, 7 - 11 May 2001, Miramare-Trieste, Ital

Coupling of Baryon Resonances to the NωN \omega channel

We estimate the resonance coupling strength $f_{RN\omega}$ and $f_{RN\rho}$ from a Vector Meson Dominance (VMD) analysis. The isoscalar and isovector part of the photon coupling are obtained separately from helicity amplitudes. The reliability of this approach is tested by comparing VMD predictions for $f_{RN\rho}$ with values obtained from fitting the hadronic decay widths into $N \rho$. A reasonable agreement is found, but VMD tends to underestimate the coupling constants. In order to confirm consistency with experimental data, we calculate the cross-sections for photon-and pion induced reactions within a {\it Breit-Wigner} model. Finally, we study how the properties of $\omega$ mesons in nuclear matter are affected from the excitation of resonance-hole loops. For an $\omega$ at rest, we find a broadening of about 40 MeV, while at higher momenta the effect of resonance excitations is reduced.Comment: 21 pages, 5 ps figures, misprints corrected, discussion added, improved calculation of gamma N -> omega N, revised version to be published in Nuclear Physics

Vector Meson Decay of Baryon Resonances

We investigate the coupling of vector mesons with nucleons to nucleon resonances in an isospin-selective VMD approach and explore the in-medium properties of vector mesons.Comment: 8 pages, 2tables, 4 figures, invited talk at NSTAR 2001, Workshop on the Physics of Excited Nucleons, University of Mainz, Germany, March 7-10, 2001. To be published in World Scientifi

Soliton surfaces associated with sigma models; differential and algebraic aspect

In this paper, we consider both differential and algebraic properties of surfaces associated with sigma models. It is shown that surfaces defined by the generalized Weierstrass formula for immersion for solutions of the CP^{N-1} sigma model with finite action, defined in the Riemann sphere, are themselves solutions of the Euler-Lagrange equations for sigma models. On the other hand, we show that the Euler-Lagrange equations for surfaces immersed in the Lie algebra su(N), with conformal coordinates, that are extremals of the area functional subject to a fixed polynomial identity are exactly the Euler-Lagrange equations for sigma models. In addition to these differential constraints, the algebraic constraints, in the form of eigenvalues of the immersion functions, are treated systematically. The spectrum of the immersion functions, for different dimensions of the model, as well as its symmetry properties and its transformation under the action of the ladder operators are discussed. Another approach to the dynamics is given, i.e. description in terms of the unitary matrix which diagonalizes both the immersion functions and the projectors constituting the model.Comment: 22 pages, 3 figure