Triggering on Leptons for Beyond the Standard Model Searches at ATLAS

This thesis presents a study of leptonic signals of possible extensions to the Standard Model at the LHC and the ability of the ATLAS detector to trigger on these. In particular, this work focuses on the effects of lepton isolation on the electron trigger at ATLAS. Monte Carlo simulations of supersymmetric processes at benchmark points and top quark decays are studied as well as dileptonic decays of Z-bosons. Differences are observed in the efficiency of the leptonic trigger between ``busy'' events, such as SUSY and top samples, and the events containing well isolated leptons commonly used to evaluate trigger efficiencies. It is shown that these differences can be explained in terms of the main kinematic properties and isolation of leptons and that a data driven efficiency measurement can be obtained using the Z-boson resonance. This method is applied across scans of supersymmetric parameter space as a weighted average, and it is shown that trigger efficiencies are high and consistent for the wide variety of models considered. Further studies examine the highly energetic electrons expected to be produced in the decays of hypothesised new heavy neutral resonances. It is shown, due to the shower structure of very energetic electrons, that isolated triggers are unsuitable for such signatures. Moreover, a data driven measurement of the electron's properties in terms of their main kinematic properties is shown to be sufficient to explain the observed efficiencies for non-isolated triggers. Finally, an initial study of the very first electrons produced in LHC collisions is presented, which displays good agreement between the data and the Monte Carlo prediction for the main kinematical variables and electromagnetic shower shapes. The efficiency of the lowest energy threshold electron trigger is measured and seen to be high and consistent with the MC prediction.