Point-Form Approach to Baryon Structure

A critical discussion is given of the results for baryon electromagnetic and axial form factors obtained from relativistic constituent quark models in the framework of Poincaré-invariant quantum mechanics. The primary emphasis lies on the point-form approach. First we summarize the predictions of the Goldstone-boson-exchange constituent quark model for the electroweak nucleon structure when using a spectator-model current in point form. Then the influences of different dynamics inherent in various kinds of constituent quark models (Goldstone-boson-exchange, one-gluon-exchange, instanton-induced interactions) are discussed. Finally the point-form results are compared to analogous predictions calculated in instant form. Relativistic effects are always of sizeable magnitude. A nonrelativistic approach is ruled out. The instant-form results are afflicted with severe shortcomings. In the spectator-model approximation for the current, only the point-form results appear to be reasonable a-priori. In fact, the corresponding quark model predictions provide a surprisingly good description of all elastic electroweak observables in close agreement with existing experimental data, specifically for the Goldstone-boson-exchange constituent quark model.