Search for excited leptons with the ATLAS detector

The Standard Model of particle physics assumes that the world consists of leptons (electrons, muons, taus and neutrinos) and quarks that are elementary particles. We measure that leptons and quarks have a mass hierarchy that could be explained by an underlying substructure of even smaller particles. Then leptons and quarks would be the lowest stable energetic states and higher energy excited states would exist and could be probed experimentally. In this thesis a search for these excited states is described using the data of the ATLAS detector at the Large Hadron Collider (LHC). The analysis is focused on excess production of events containing at least 3 Standard Model leptons that could indicate the presence of excited states. The results of the search for all 6 lepton flavors ($e^*$, $\mu^*$, $\tau^*$, $\nu_e^*$, $\nu_\mu^*$, $\nu_\tau^*$) using the 2011 and 2012 ATLAS data sets are presented and perspectives for the next LHC run which started 2015 are shown. Since no hints for excited lepton states were found, limits were set on the mass of the excited leptons and the compositeness scale, $\Lambda$. For excited taus and excited neutrinos these limits are the first limits set by any LHC experiment.