Search for supersymmetric particles in final states with jets and missing transverse momentum with the ATLAS detector

The Standard Model of particle physics (SM) is very successful in describing elementary particles and their interactions. The recent discovery of a new boson at the LHC continues this successful story as it is compatible with the last undiscovered particle in the SM, the Higgs boson. However, the SM has limitations such as the hierarchy problem or the missing dark matter candidate. One of the extensions to the SM includes a new space-time symmetry, called Supersymmetry (SUSY), resulting in a symmetry between fermions and bosons. In most phenomenological SUSY models the production of supersymmetric particles at the LHC is dominated by squark-squark, squark-anti-squark, squark-gluino and gluino-gluino pair production. Squarks are the super-partners to quarks and gluinos the super-partners to the gluons. These particles decay subsequently into the lightest supersymmetric particle which does not interact with detector material. Thus the striking signature for such a pair production of supersymmetric particles in proton-proton collisions are multiple jets in combination with missing transverse energy. This thesis contains a search for supersymmetric particles in final states with jets and missing transverse momentum on data collected by the ATLAS experiment. The analysis corresponding to an integrated luminosity of 4.7 inverse femtobarn of 7 TeV center of mass energy proton-proton collisions is discussed in detail. The search is carried out in dedicated signal regions defined in inclusive jet multiplicities from two to six jets and a large missing transverse momentum component. The dominant sources of background in the search are as follows. The W + jets background is composed of events with a W decaying to a tau and a neutrino or a W decaying into an electron or muon in which no electron or muon candidate is reconstructed. The irreducible background Z + jets is dominated by events with a Z decaying to neutrinos and thus with large missing transverse energy. Hadronic decays in t¯t-quark events which decay semi-leptonically and single top events can generate large missing transverse momentum and pass the jet and lepton requirements. The multi-jet background is caused by rare instances of poor reconstruction of jet energies in calorimeters leading to fake missing transverse momentum and by neutrinos in the semi-leptonic decay of heavy quarks. For each of the main background components a control region was defined which requirements are as close as possible to the signal region selection, thus minimizing systematic uncertainties arising from extrapolation from control regions to the signal regions. For each selection a simultaneous normalization of the backgrounds is performed, taking into account correlations in the systematic uncertainties and controlling signal contamination to the control regions. No deviation from the SM expectation has been found in the data. Stringent limits on the masses of gluinos and the first two generations of squarks are presented, which exceed the limits set by previous experiments.