Estimation of the $Z\to\nu\nu$ background to New Physics searches in ATLAS

This thesis describes a series of studies related to searches for new phenomena, beyond the Standard Model of particle physics, in high energy hadron collisions. In such searches, it becomes crucial to identify the Standard Model backgrounds in order to resolve a potential new signal. The thesis presents a method that uses photon events to determine one of such backgrounds, caused by the production of $Z$ boson events. The studies performed to validate the method, both theoretically and experimentally, are presented and the method was shown to be successful as well as to provide reliable results. Theoretically, the method is found to be robust up to a ~10\% uncertainty. Experimentally, the method is implemented to estimate the $Z (\to\nu\nu)+\mathrm{jets}$ background for the SUSY $0l+ E_T^{\mathrm{miss}}+\mathrm{jets}$ search in the ATLAS experiment at the Large Hadron Collider, where this background is one of the most important components for the final sensitivity and is impossible to measure directly. The main experimental results presented are the latest from ATLAS at the time of writing, corresponding to the full dataset of proton-proton collisions delivered by the LHC in 2011 (4.7 fb^-1) at $\sqrt{s}=7$ TeV. Given that this method has been mainstream since 2010, brief comparisons to the results from previous analyses that used smaller datasets with the same $\sqrt{s}$ are also given, as well as additional cross-checks that support the robustness and validity of the method. The results presented here have contributed to the determination of the world's best limits with respect to SUSY models, which currently exclude equal mass squarks and gluinos below 1.4 TeV.