GSI-Kolloquium: Fundamental physics with ultra-cold neutrons

Ultra-cold neutrons (UCN) are a unique tool to probe fundamental properties of nature and the early Universe without the use of accelerators. In this talk I will present an overview of selected highlights in this field of research, e.g. quantum-optical experiments like a trap for high-field seeking spin states, as well as investigations of gravity at short distances with new spectroscopic methods. A focus will be a flagship measurement: the search for the time-reversal-symmetry violating permanent electric dipole moment (EDM) of the neutron. Such a phenomenon is required in most theories to explain the excess of matter versus antimatter in the Universe, a problem which the standard model of particle physics fails to explain by many orders of magnitude. The search for EDMs is already ongoing for many years in different systems, it has already ruled out or restricted many theories and already probes energy scales beyond the reach of the LHC. With the new approach currently being realized at the FRM-2 reactor in Garching, the sensitivity will be further increased by two orders of magnitude down to 10^-28 ecm. This requires next to the strongest possible source of UCN also an elaborate effort to control systematic effects in small magnetic- and large electric fields.