Demonstration of SU(2)-symmetry by neutron interferometry

Neutron interferometry provides a direct test of the 4π-symmetry of a fermion wave function. The experiments performed with perfect crystal interferometers to demonstrate that SU(2)-symmetry property are reviewed. The measured periodicity value of 716.8 +- 3.8 degrees, which is the most precise one obtained up to now, is in good agreement with theoretical prediction. Effects are discussed which may give rise to deviations of the experimental result from the 4π value. Furthermore, an account of epistemological aspects is given relating to the question of the operational applicability of the term rotation in the interpretation of the experiments. Finally, proposals for new, more precise, experiments are made. Some of these experiments may have particular relevance in the context of considerations of a breaking of SU(2) symmetry under strong interaction