Marco Meineri (EPFL Lausanne) Simone Giacomelli ((ICTP Trieste))
In a quantum or statistical field theory, the action of operators on the vacuum describes how a physical system is affected by a probe. Often, the probes are extended objects: the word-line of a heavy particle, a boundary or an interface, etc. We describe some general features of extended probes, in highly symmetric QFTs. We first focus on conformal field theories. We discuss properties of the Operator Product Expansion in the presence of defects and give examples of topological and non topological conformal defects. We then move to the case of supersymmetric theories, in which we can construct defects with two-dimensional (or higher) support. We explain how certain kinds of extended probes are able to detect the phases of the theory. Finally, we describe the relevance of defects in constructing a large class of superconformal N=2 theories in four dimensions, and how one can use properties of topological defects in two dimensions to compute the expectation value of line operators in these theories.