Universal three- and four-body phenomena in an ultracold gas of cesium atoms

This thesis presents experimental studies of universal three- and four-body phenomena performed in an ultracold quantum gas of cesium atoms by trap-loss spectroscopy. The main part of this thesis examines the Efimov scenario, which predicts the existence of an infinite series of universally related three-body bound states for resonant two-body interactions. These trimer states couple to the free-atom continuum and to the atom-dimer threshold. Both situations lead to experimentally detectable enhancements of the particle loss, which are referred to as tri-atomic and atom-dimer Efimov resonances, respectively. We obtain the first evidence of an atom-dimer Efimov resonance in an ultracold atom-dimer mixture. In another experiment, the properties of the three-body parameter, which incorporates all unknown short-range interactions in universal theories, are studied in an atomic cesium gas. The analysis of our findings shows that the three-body parameter stays essentially constant for the scattering scenarios investigated. Utilizing an ultracold atom-dimer sample, we observe a magnetically controllable exchange process. This experiment represents the first demonstration of an elementary reaction process in the regime of universal interactions. Universal four-body processes in pure dimer and atom samples are another focus of research covered in this thesis. A collisional study based on universal dimers reveals a pronounced loss minimum. Furthermore, we analyze four-body losses in an ultracold atomic sample, thereby identifying two resonances representing the first evidence for the existence of universal four-body states, which are tied to an Efimov trimer. (author)