Study of WW scattering in the absence of a light Higgs boson using the ATLAS Detector at the LHC

The Thesis investigates the possible case that the Electrow eak Symmetry Breaking is not due to a light, weakly interacting Higgs boson, but inste ad 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 c onstruct a low-energy effective theory for the electroweak interactions and using the Pad ́e unitarisation protocol, certain resonances can be predicted. The scattering of long itudinally 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 ma ss range of 600 GeV up to 2.4 TeV and for different resonance scenarios, includin g 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 hadron ic environment, the contribution from the background processes can be reduced in such a way tha t the ATLAS detector will be able to see interesting WW scattering events within the ab ove mass range with 30 f b − 1 of data and it will be also possible to distinguish among the d ifferent resonance scenarios. A key study of the present research is the measurement of the W W scattering cross-section and it has been calculated that a significant measurement wil l be achieved, for the most pessimistic scenario, with 80 f b − 1 of data. Regarding the performance of the ATLAS detector, the Thesis focuses on the Level-2 Trigger for selecting interesting physics events. After a s hort 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 ba sed on the information from the Electromagnetic Calorimeter and it will be demonstrated th at 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 Detecto r, especially during the high luminosity phase of the LHC