Multipole Form Factors Research Articles

The kinematic and symmetry structure of e +e −→hadron pairs

The kinematic singularity and constraint free amplitudes for e +e −→two particles which can have arbitrary spins are obtained in a simple closed form. The relationships of these amplitudes with the usual multipole form factors are given. It is noted that SU(6) W and tests of ideas based on the Melosh transformation are liberated from their γN and πN arena and can be confronted with arbitrary targets and axcitations. The matrix elements or arbitrary meson pairs are then derived, rates are computed for pseudoscalar and vector meson production, and some tests of symmetry breaking mechanisms proposed. Extension to SU(4)[SU(8) W] broken in the masses enables estimates of charmed particle production rates to be obtained. General features of πR final states suggest that scaling violation at small x may be connected with threshold phenomena as channels open up. A subtle difference between space-like and time-like regions is highted.

Multipole γN- Δ form factors and resonant photo- and electroproduction

Multipole form factors for the γN- Δ vertex are defined explicitly as the coefficients of appropriate kinematic covariants. The relations with other decompositions of the vertex are fully explored and a complete set of angular distribution and cross-section formulae derived. The experimental and theoretical status of the magnetic dipole form factor is briefly reviewed.

Polarization effects in photonuclear reactions

We derive explicit formulae for angular asymmetries and longitudinal and transverse polarizations of nucleons emitted from nuclei in the (γ, N) reaction, corresponding to linearly or circularly polarized as well as unpolarized incident photons. The reaction is assumed to proceed in two stages, namely, by photo-excitation of a single nuclear level and its subsequent decay via nucleon emission. Our expressions are given in terms of the nuclear transverse multipole form factors for the excitation and of the reduced matrix elements corresponding to the decay. The results are illustrated by obtaining nucleon angular distributions and transverse polarizations following photo-excitation of the giant dipole resonance in 16O, using Gillet's model and R-matrix theory. The comparison of these results with experiment is satisfactory.