QCD background estimation for Supersymmetry searches with jets and missing transverse momentum with the ATLAS experiment at the Large Hadron Collider

Some of the most interesting questions mankind might ask are closely related to the field of astro- and particle physics: What are the fundamental building blocks of our universe and how do they interact? Will there eventually be a theory that can describe everything? During the last decades, particle collision experiments unraveled various aspects of these mysteries - and a very successful theory emerged: the Standard Model (SM) of particle physics: As of today’s knowledge, matter consists of fermions, the quarks and the leptons. Among these, four fundamental interactions are known: the strong, the weak, the electromagnetic and the gravitational force. These interactions are mediated by bosons (force carriers). While the SM describes the first three interactions with high precision, various fundamental questions remain unanswered - such as the structure in the SM itself, the origin of dark matter/energy and the matter-/antimatter asymmetry. One aesthetically appealing solution is Supersymmetry (SUSY), which imposes an additional internal symmetry between bosons and fermions and thus predicts the existence of a partner for each fundamental particle with the same quantum numbers but different spin. The Large Hadron Collider (LHC) at CERN is designed to collide protons at a nominal centre-of-mass energy of sqrt(s) = 14 TeV and hosts i.a. two multipurpose experiments, ATLAS and CMS. In order to detect Supersymmetric particles at hadron colliders, various efforts have been made to develop significant analyses providing the largest possible discovery reach. Hereby, one of the most powerful channels examines the signature of fully hadronic final states combined with large missing transverse momentum (MET), originating from the stable lightest supersymmetric particle escaping the detectors unseen. A key ingredient for any searches for new physics, however, is the precise knowledge both of the detector performance and the background determination, as classically the data are scanned for deviations from the SM predictions. At hadron colliders, the fully hadronic final state naturally suffers from a huge QCD multijet background, which is to be suppressed by several orders of magnitude by a dedicated event selection, leaving the probability for QCD multijet events to pass any of the signal region cuts by design to be small. Nevertheless, the low acceptance could be outmatched by the large QCD cross-section, leading to a significant contamination of the signal regions: As typical QCD multijet events possess only few real missing energy, additional sources of MET are needed, e.g. from leptonic decays of heavy quarks producing neutrinos or only apparent missing transverse energy from mismeasurements. Thus, the estimation of the QCD background is crucial and one of the most challenging backgrounds to determine, as the conventional use and the reliability of the MC simulation is limited (mainly due to statistics reasons) and a very detailed detector understanding, especially of the modelling of MET, would be required. At ATLAS, SUSY searches in general use transfer factors (TFs) to derive the background estimates from several control regions, whereas the TFs are mainly determined from MC simulations and validated in control regions with data. This thesis discusses several possibilities to estimate the QCD background for SUSY searches with jets and MET, also showing the limitations of the early mainly MC-based approaches. As a remedy, a new semi-data-driven method has been developed estimating the QCD background by a direct measurement of the TF. An important variable hereby is found to be the minimum angular separation between the jets and the MET-vector, providing good distinction power between QCD and non-QCD processes. The QCD TF, i.e. the ratio of events in a QCD-enhanced and a QCD-suppressed region, is evaluated as function of MET in two different control regions, defined using the regime of low MET and another key variable, commonly used for QCD suppression, namely the ratio MET/Meff, where the effective mass, Meff, is the sum of the jets’ transverse momenta and MET. Correcting the observed event counts in the data for the non-QCD contamination, the QCD TFs are fitted and the final numbers are extracted from a interpolation of the fits into the signal regions. This thesis provides also a comprehensive estimation of the systematic uncertainties on the TFs, arising e.g. from jet energy scale uncertainty and pile-up, the overlay of several hard interactions within one event. The obtained results of the different methods are compared with each other and with the baseline method, which has been used in previous ATLAS publications so far. Finally, this thesis shows how the obtained QCD TFs can be combined with other background estimates to set competitive exclusion limits on the allowed SUSY particle masses, in case no significant deviations from the Standard Model are found.