Electromagnetic moments of relativistic higher spin baryons

We point out a source of ambiguities in the measurements of the electromagnetic moments of spin-3/2 baryons which rely on relativistic Lagrangian models. An anambiguous relation between the parameters of the spin-3/2 electromagnetic Lagrangian and the electromagnetic moments of the spin-3/2 particle exits in general only for on-shell situation, while the measurements are done on ``virtual'' baryons.Comment: 4 pages, 2 figures; contribution to the proceedings of the International Conference on Quark Nuclear Physics (Adelaide, February 2000

New symmetry current for massive spin-3/2 fields

We present several new results which will be of value to theorists working with massive spin-3/2 vector-spinor fields as found, for example, in low and intermediate energy hadron physics and also linearized supergravity. The general lagrangian and propagator for a vector-spinor field in d-dimensions is given. It is shown that the observables of the theory are invariant under a novel continuous symmetry group which is also extended to an algebra. A new technique is developed for exploring the consequences of the symmetry and a previously unknown conserved vector current and charge are found. The current leads to new interactions involving spin-3/2 particles and may have important experimental consequences.Comment: 9 pages, references updated and minor change

Relativistic covariance of quasipotential equations

We argue that most of the relativistic 3-D (quasipotential) equations used in hadron physics are inconsistent with the discrete symmetries like charge conjugation and CPT, yielding an incorrect Lorentz structure for the calculated Green's functions. An exception to this is the equal time approximation to the Bethe-Salpeter equation. We present a covariant quasipotential model for the pion-nucleon interaction based on hadronic degrees of freedom and satisfying the full covariance.Comment: 7 pages, LaTeX (REVTeX), incl. 4 PostScript figure

Causality in the relativistic bound-state problem

Although the exact Bethe-Salpeter equation is certainly the appropriate field-theoretic framework to describe the non-perturbative problem of scattering and bound states, the inevitable truncations introduce inconsistencies such as loss of symmetries or incorrect one-body limit. I conjecture that these problem can be overcome if the truncation preserves the field-redefinition invariance of the exact equation. A sum rule for light-by-light scattering can provide a testing ground of this conjecture.Comment: 4 pp, 2 figs; Contribution to the 5th Asia-Pacific Conference on Few-Body Problems in Physics (APFB11

Model-independent effects of Delta excitation in nucleon polarizabilities

Model-independent effects of $\Delta$(1232) excitation on nucleon polarizabilities are computed in a Lorentz-invariant fashion. We find a large effect of relative order $(M_\Delta - M)/M$ in some of the spin polarizabilities, with the backward spin polarizability receiving the largest contribution. Similar subleading effects are found to be important in the fourth-order spin-independent polarizabilities $\alpha_{E\nu}$, \al_{E2}, \be_{M\nu}, and \be_{M 2}. Combining our results with those for the model-independent effects of pion loops we obtain predictions for spin and fourth-order polarizabilities which compare favorably with the results of a recent dispersion-relation analysis of data.Comment: 14 pages, 3 figs, 4 tables; substantial revision, calculation of higher order polarizabilities include