Secret Symmetries And Supersymmetry: Investigations

This is a report of the author's research in the fields of Non-linearly Realized Spacetime Symmetries and Phenomenology of Supersymmetry. In the first part, the equivalence of two theories that non-linearly realize a spacetime symmetry, and which are related by a well motivated mapping, is discussed (with focus on the so-called Galileon group). This is done by studying how their coupling with gravity changes under the mapping. The second part treats some aspects of the phenomenology of the supersymmetric partner of the goldstino (the sgoldstino) in the context of Gauge Mediation. The work includes introductory sections on the two subjects.