Generalized Polarizabilities in a Constituent Quark Model

We discuss low-energy virtual Compton scattering off the proton within the framework of a nonrelativistic constituent quark model. A simple interpretation of the spin-averaged generalized polarizabilities is given in terms of the induced electric polarization (and magnetization). Explicit predictions for the generalized polarizabilities obtained from a multipole expansion are presented for the Isgur-Karl model and are compared with results of various models.Comment: 10 pages, Latex, 2 figures, uses ws-p8-50x6-00.cls, talk given at NSTAR2001, Workshop on "The Physics of Excited Nucleons," Mainz, Germany, March 7-10, 200

Pion polarizabilities: No conflict between dispersion theory and ChPT

Recent attempts to determine the pion polarizability by dispersion relations yield values that disagree with the predictions of chiral perturbation theory. These dispersion relations are based on specific forms for the absorptive part of the Compton amplitudes. The analytic properties of these forms are examined, and the strong enhancement of intermediate-meson contributions is shown to be connected to non-analytic structuresComment: 9 pages, 4 figures; Proceedings of 6th International Workshop on Chiral Dynamics, 6-10 July 2009, Bern, Switzerlan

Collective correlations in C12

The strong coupling of the giant resonance to the surface vibrations in C12 results in the splitting of the single one-particle, one-hole, 1- collective state into several components, thus improving the agreement between theory and experiment to a very large extent

Dispersion relation formalism for virtual Compton scattering and the generalized polarizabilities of the nucleon

A dispersion relation formalism for the virtual Compton scattering (VCS) reaction on the proton is presented, which for the first time allows a dispersive evaluation of 4 generalized polarizabilities at a four-momentum transfer $Q^2 \leq$ 0.5 GeV$^2$. The dispersive integrals are calculated using a state-of-the-art pion photo- and electroproduction analysis. The dispersion formalism provides a new tool to analyze VCS experiments above pion threshold, thus increasing the sensitivity to the generalized polarizabilities of the nucleon.Comment: 4 pages, 2 figure

Collective correlations in spherical nuclei and the structure of giant resonances

The theory of collective correlations in nuclei is formulated for giant resonances interacting with surface vibrations. The giant dipole states are treated in the particle-hole framework, while the surface vibrations are described by the collective model. Consequently, this treatment of nuclear structure goes beyond both the common particle-hole model (including its various improvements which take ground-state correlations into account) and the pure collective model. The interaction between giant resonances and surface degrees of freedom as known from the dynamic collective theory is formulated in the particle-hole language. Therefore, the theory contains the particle-hole structures and the most important "collective intermediate" structures of giant resonances. Detailed calculations are performed for 12C, 28Si, and 60Ni. A good detailed agreement between theory and experiment is obtained for all these nuclei, although only 60Ni is in the region where one would expect the theory to work well (50< A <110)

Dispersion analysis for generalized spin polarizabilities

We report on a dispersion relation formalism for the virtual Compton scattering (VCS) reaction on the proton, which for the first time allows a dispersive evaluation of 4 generalized polarizabilities. The dispersion formalism provides a new tool to analyze VCS experiments above pion threshold, thus increasing the sensitivity to the generalized polarizabilities of the nucleon.Comment: 5pages, 2 figures, to appear in the Proceedings of the Symposium on the Gerasimov-Drell-Hearn Sum Rule and the Spin Structure in the Nucleon Resonance Region (GDH2000), June 14-17 2000, Mainz, German

Higher order forward spin polarizability

As a guideline for future experiments to extract the four (leading) spin polarizabilities of the nucleon, we have constructed the forward amplitude for polarized Compton scattering by dispersion integrals. These integrals have been saturated by recently measured helicity-dependent photoabsorption cross sections as well as predictions for pion photoproduction multipoles from several phenomenological descriptions and chiral perturbation theory. The comparison of these results corroborates the strategy to extract the spin polarizabilities by fitting them to polarized Compton data and fixing all higher order spin effects by dispersion relations based on pion photoproduction multipoles.Comment: 21 pages, 6 figures, 3 Tables; version to appear in Phys. Lett.

Dispersion relation formalism for virtual Compton scattering off the proton

We present in detail a dispersion relation formalism for virtual Compton scattering (VCS) off the proton from threshold into the $\Delta(1232)$-resonance region. Such a formalism can be used as a tool to extract the generalized polarizabilities of the proton from both unpolarized and polarized VCS observables over a larger energy range. We present calculations for existing and forthcoming VCS experiments and demonstrate that the VCS observables in the energy region between pion production threshold and the $\Delta(1232)$-resonance show an enhanced sensitivity to the generalized polarizabilities.Comment: 51 pages, 15 figure

The Counting of Generalized Polarizabilities

We demonstrate a concise method to enumerate the number of generalized polarizabilities---quantities characterizing the independent observables in singly-virtual Compton scattering---for a target particle of arbitrary spin s. By using crossing symmetry and J^{PC} conservation, we show that this number is (10s+1+delta_{s,0}).Comment: 10 pages, revtex4, no figures. Version to appear in Phys. Rev. D. Paper now divided into sections and clarifying comments added, but physics content unchange