A search for evidence that fundamental particles are made of smaller subconstituents is performed. The existence of excited states of fundamental particles would be an unambiguous indication of their composite nature. Experimental signatures compatible with the production of excited states of charged leptons in electron-positron collisions are studied. The data analysed were collected by the OPAL detector at the LEP collider. No evidence for the existence of excited states of charged leptons was found. Upper limits on the product of the cross-section and the electromagnetic branching fraction are inferred. Using results from the search for singly produced excited leptons, upper limits on the ratio of the excited lepton coupling constant to the compositeness scale are calculated. From pair production searches, 95% confidence level lower limits on the masses of excited electrons, muons and taus are determined to be 103.2 GeV.

Our search for compositeness of electrons and muons is presented using a data sample of proton-proton collisions at a center-of-mass energy of √(s) = 8 TeV collected with the CMS detector at the LHC and corresponding to an integrated luminosity of 19.7 fb-1. Excited leptons (l*) produced via contact interactions in conjunction with a standard model lepton are considered, and a search is made for their gauge decay modes. The decays considered are l* →l? and l* → lZ, which give final states of two leptons and a photon or, depending on the Z-boson decay mode, four leptons or two leptons and two jets. The number of events observed in data is consistent with the standard model prediction. Exclusion limits are set on the excited lepton mass, and the compositeness scale L. For the case Ml* = L the existence of excited electrons (muons) is excluded up to masses of 2.45 (2.47) TeV at 95% confidence level. The neutral current decays of excited leptons are considered for the first time, and limits are extended to include the possibility that the weight factors f and f ', which determine the couplings between standard model leptons and excited leptons via gauge mediated interactions, have opposite sign.

A search for excited leptons is carried out with the CMS detector at the LHC, using 36 inverse picobarns of pp collision data recorded at sqrt(s) = 7 TeV. The search is performed for associated production of a lepton and an oppositely charged excited lepton pp to l l*, followed by the decay l* to l gamma, resulting in the l l gamma final state, where l = electron or muon. No excess of events above the standard model expectation is observed. Interpreting the findings in the context of l* production through four-fermion contact interactions and subsequent decay via electroweak processes, first upper limits are reported for l* production at this collision energy. The exclusion region in the compositeness scale Lambda and excited lepton mass M(l*) parameter space is extended beyond previously established limits. For Lambda = M(l*), excited lepton masses are excluded below 1070 GeV/c2̂ for e* and 1090 GeV/c2̂ for mu* at the 95% confidence level.

This dissertation presents two searches for phenomena beyond the Standard Model using events with three or more charged leptons. The searches are based on 20.3 fb-1 of proton- proton collision data with a center-of-mass energy of √s = 8 TeV collected by the ATLAS detector at the CERN Large Hadron Collider in 2012. The first is a model-independent search for excesses beyond Standard Model expectations in many signal regions. The events are required to have least three charged leptons, of which at least two are electrons or muons, and at most one is a hadronically decaying [tau] lepton. The selected events are categorized based on the flavor and charge of the leptons, and the signal regions are defined using several kinematic variables sensitive to beyond the Standard Model phenomena. The second search looks for new heavy leptons decaying resonantly to three electrons or muons, two of which are produced through an intermediate Z boson. The resonant decay produces a narrowly- peaked excess in the trilepton mass spectrum. In both cases, no significant excess beyond Standard Model expectations is observed, and the data are used to set limits on models of new physics. The model-independent trilepton search is used to confront a model of doubly charged scalar particles decaying to e[tau] or [mu][tau], excluding masses below 400 GeV at 95% confidence level. The trilepton resonance search is used to test models of vector-like leptons and the type III neutrino seesaw mechanism. The vector-like lepton model is excluded for most of the mass range 114 GeV - 176 GeV, while the type III seesaw model is excluded for most the mass range 100 GeV - 468 GeV. Both searches also present tools to facilitate reinterpretations in the context of other models predicting the production of three or more charged leptons.

This was the most recent in a highly esteemed series of biannual Rochester conferences. 20 invited reviews and about 200 invited contributions on all aspects of current research in high energy and particle physics give a complete and lively account of achievements, activities and goals in the field. Topics discussed include results from proton-antiproton and electron-positron colliders, spectroscopy and decays of heavy flavors, weak mixing and CP violation, non-accelerator particle physics, heavy ion collisions, future accelerators, detector developments, the standard electroweak model and beyond, the status of perturbative QCD, superstrings and unification, new developments in field theory, non-perturbative methods, and cosmology and astrophysics.