Stefanidis, E; (2008) Study of the WW scattering in the absence of light Higgs boson using the ATLAS detector at LHC. Doctoral thesis , UCL (University College London).

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Abstract

The Thesis investigates the possible case that the Electroweak Symmetry Breaking is not due to a light, weakly interacting Higgs boson, but instead the Symmetry is broken by strongly interacting heavy resonances. In that case, the formalism of the Electroweak Chiral Lagrangian can be used as a model-independent way to construct a low-energy effective theory for the electroweak interactions and using the Pade unitarisation protocol, certain resonances can be predicted. The scattering of longitudinally polarized W bosons, one of which decays leptonically and the other hadronically, is used as a channel to probe the mechanism of the Electroweak Symmetry Breaking in the mass range of 600 GeV up to 2.4 TeV and for different resonance scenarios, including the case that there is no resonance in the spectrum. The reconstruction of the vector bosons is described and it is demonstrated that, by exploiting key features of the hadronic environment, the contribution from the background processes can be reduced in such a way that the ATLAS detector will be able to see interesting WW scattering events within the above mass range with 30 fb x of data and it will be also possible to distinguish among the different resonance scenarios. A key study of the present research is the measurement of the WW scattering cross-section and it has been calculated that a significant measurement will be achieved, for the most pessimistic scenario, with 80 fb l of data. Regarding the performance of the ATLAS detector, the Thesis focuses on the Level-2 Trigger for selecting interesting physics events. After a short introduction to the Trigger system of the detector, the concept of the Region of Interest is explained and a method for reducing its geometrical size is presented. The method is based on the information from the Electromagnetic Calorimeter and it will be demonstrated that the reduced size can improve the performance of the Level-2 Trigger in terms of execution time (58% faster) but also for track reconstruction (6% more efficient) in the Inner Detector, especially during the high luminosity phase of the LHC.

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