Affiliation: | (1) School of Mathematics, Institute for Advanced Study, 08540 Princeton, NJ, USA;(2) Theoretische Physik, Universität Zürich, Schönberggasse 9, CH-8001 Zürich, Switzerland;(3) Present address: Theoretische Physik, ETH-Hönggerberg, CH-8093 Zürich, Switzerland;(4) Istituto Nazionale di Fisica Nucleare, Sezione di Pavia, Italy |