Meson fields, exchange currents and progenitor sum rules

The theory of the integrated photoabsorption cross section and the dipole sum rule is reviewed. A new progenitor sum rule, in which meson fields appear explicitly, is derived from a field theory of interacting pions and nucleons. The relation of this sum rule to previous results obtained in a potential theory is elucidated, and the role of neutron-proton correlations in nuclear ground states is discussed in terms of their effect on the dipole sum rule. The inclusion of ρ-mesons (again in an oversimplified theory) and their contribution to progenitor sum rules is also mentioned.     

Self-consistent treatment of one-gluon exchange in the soliton bag model

In the context of the soliton bag model, the quarks, soliton field and the gluon propagator are calculated self-consistently in the mean-field, one-gluon exchange approximation. The use of a confined propagator, as compared with a free propagator, is primarily to change the effective quark-gluon coupling constant by a factor of approximately two in the case of static, spherical bags. Self-consistency leads to only a small improvement in the fit to physical data, after readjustment of model parameters. Nevertheless, use of a confined propagator is expected to have an important effect on deformed bags and in dynamical calculations.     

Meson exchange currents for nuclear muon capture by3He leading to the triton final state

We have calculated exchange corrections for nuclear muon capture by³He using the hard-pion model and realistic nuclear wavefunctions. These currents modify the vector and axial part of the weak nuclear current. In their absence the rate is found to be about 12% smaller than experiment. Our aim is to test the hard-pion model by making a realistic estimate of the theoretical uncertainty and comparing the calculated rate with its experimental value. Results for the rate and spin observables will be presented at the symposium. The recent precise measurement of the rate (see the contributions of A.A. Vorobyov and W. Prymas in these proceedings) allows the most stringent test of exchange currents for muon capture to date.Supported by the Sir James Knott Fellowship.Supported by grants nos. 202/94/0370 (GA CR) and 148410 (GA ASC CR).     

Composite-meson model and meson exchange currents

We study the meson exchange currents (MEC) mechanism for the pion double-charge exchange (DCX) reaction in a composite-meson model. The model assumes that the mesons are two-quark systems and can interact with each other only through quark loops. The contributions of the ρ, σ, and f₀ mesons, the four-quark box diagram as well as a contact diagram has been taken into account. It is shown that the contribution of the ρ, σ, and f₀ mesons increases the forward scattering cross-section in an average by 25% and decreases with energy.     

Canonical quantization of the Skyrme model

The canonical quantization of the Skyrme model is presented. Due to the complexities of the original set-up, first postulated by Rajeev, a new set of fields is found which satisfy a Lie algebra and for this reason, simplifies the study of the system. Using the new fields, the properties of the states under rotations are examined. As a result we re-derive the quantization of the coefficient of the Wess-Zumino action as an a priori consistency condition. We also find under what circumstances states with non-vanishing winding number behave like fermions.