Electroweak Processes in Few-Nucleon Systems

After a brief introduction into the basic ingredients of electroweak theory as a spontaneously broken local, non-Abelian gauge symmetry, the general properties of the electromagnetic current and two-photon operators are discussed. The specific forms of vector and axial one-nucleon currents are given, together with lowest order $\pi$ meson exchange and isobar currents as well as meson production currents. A brief overview is given on the most important one- and two-boson processes. Electron scattering in the one-boson- approximation is then considered in greater detail. Formal expressions of the cross section for inclusive and exclusive processes are given, including parity violating contributions from $\gamma$-$Z$ interference as well as from parity violating components in the hadronic wave function. Specific electromagnetic reactions on the deuteron are then discussed with respect to the influence of meson exchange currents, isobar configurations in the deuteron groundstate, relativistic contributions and the role of $\pi$-meson retardation. Furthermore, recent results on coherent and incoherent $\pi$- and $\eta$-photoproduction are presented as well as a discussion of the Gerasimov-Drell-Hearn sum rule and the effect of a parity violating deuteron component on inclusive electron scattering off the deuteron for quasifree kinematics. The review closes with a summary and a brief outlook.Comment: 56 pages latex including 42 postscript figures, Lectures held at the Int. Workshop on Few-Body Problems in Nuclear Physics and Related Fields, ECT$^\ast$, Trento, Italy, September 8-27, 199

The GDH sum rule for the deuteron

As first topic, the GDH sume rule is discussed in the context of a more general class of sum rules associated with the various contributions to the total photoabsorption cross section for target and beam polarization. Then I address the question of whether the GDH sum rule for the neutron can be determined from the one for the deuteron. It appears that this will not be possible in a simple manner. The spin response of the deuteron is calculated including contributions from the photodisintegration channel and from coherent and incoherent single pion production as well, and the GDH integral is evaluated up to a photon energy of 550 MeV. The photodisintegration channel converges fast enough and gives a large negative contribution, essentially from the $^1S_0$ state near threshold and its absolute size is about the same than the sum of proton and neutron GDH values. It is only partially cancelled by the single pion production contribution. But the incoherent channel has not reached convergence at 550 MeV.Comment: talk at GDH-2000 Symposium, Mainz, 14.-17.6.2000 12 pages latex including 3 figures, needs world scientific style (ws-p8-50x6-00.cls

Coulomb effects in polarization transfer in elastic antiproton and proton electron scattering at low energies

The influence of Coulomb distortion on the polarization transfer in elastic proton and antiproton electron scattering at low energies is calculated in a distorted wave Born approximation. For antiproton electron scattering Coulomb effects reduce substantially the spin transfer cross section compared to the plane wave Born approximation whereas for proton electron scattering they lead to a dramatic increase for kinetic proton lab energies below about 20 keV.Comment: 10 pages, 7 figures Few typos corrected, accepted for publication in Eur. Phys. J. A, few typos corrected, last sentence of conclusions change

Analysis of the low-energy η\etaNN-dynamics within a three-body formalism

The interaction of an $\eta$-meson with two nucleons is studied within a three-body approach. The major features of the $\eta NN$-system in the low-energy region are accounted for by using a s-wave separable ansatz for the two-body $\eta N$- and $NN$-amplitudes. The calculation is confined to the $(J^\pi;T)=(0^-;1)$ and $(1^-;0)$ configurations which are assumed to be the most promising candidates for virtual or resonant $\eta NN$-states. The eigenvalue three-body equation is continued analytically into the nonphysical sheets by contour deformation. The position of the poles of the three-body scattering matrix as a function of the $\eta N$-interaction strength is investigated. The corresponding trajectory, starting on the physical sheet, moves around the $\eta NN$ three-body threshold and continues away from the physical area giving rise to virtual $\eta NN$-states. The search for poles on the nonphysical sheets adjacent directly to the upper rim of the real energy axis gives a negative result. Thus no low-lying s-wave $\eta NN$-resonances were found. The possible influence of virtual poles on the low-energy $\eta NN$-scattering is discussed.Comment: 16 pages revtex including 10 figure

Interaction of η\eta mesons with a three-nucleon system

A study of the $\eta$-3N interaction in the energy region near the $\eta$-^3H elastic scattering threshold is presented. The calculational scheme is based on the four-body scattering formalism. A manageable form of the integral equations is achieved by using a separable ansatz for the driving two-body $\eta N$- and NN-forces as well as for the subamplitudes appearing in the (1+3) and (2+2) partitions of the $\eta$-3N system. Results presented for the $\eta$-^3H scattering length point to the existence of a virtual (antibound) s-wave $\eta$-^3H state. As a consequence, a large enhancement of the cross section for $\eta$-production from three-body nuclei is predicted.Comment: 4 pages including 2 eps-figures, Contribution to MESON 2002, 7th Int. Workshop on Production, Properties and Interaction of Mesons, Cracow, Poland, May 24-28, 200