Search for Physics beyond the Standard Model in Events with a Z Boson and Large Missing Transverse Momentum with the ATLAS Detector.

Abstract

This dissertation summarizes two searches for new physics in LHC proton collision data collected by the ATLAS detector at CERN. In particular, these searches were designed to optimize the chances of discovery of supersymmetric particles, assuming a supersymmetry breaking scheme known as General Gauge Mediation (GGM). The final state considered in these analyses consists of a Z boson, where the Z decays to an electron or muon pair, in association with large missing transverse momentum and jets. The first of the two analyses is based on data collected in 2011, when the LHC delivered collisions at a center of mass energy 7 TeV. Using 1.04 fb−1 of good quality ATLAS data, signal region optimization and quantification of backgrounds using data-driven methods were carried out. No excess above the Standard Model expectation was observed, and these results were interpreted in a GGM context in which the lightest neutralino (the NLSP) is higgsino-like. A follow-up to this search was also performed using 5.84 fb−1 of 8 TeV ATLAS data recorded in 2012. Overall, this analysis is similar to the 2011 work, with some changes in the data-driven background methods and signal models used for interpretation. Again, no excess was observed in relation to the expectation from the Standard Model processes. In addition to the GGM models used in 2011, which are characterized with a low value of tan(beta), a high tan(beta) interpretation was considered for the 2012 analysis. 95% CL limits on the gluino and higgsino mass have been set. For the low tan(beta) scenario, gluino masses in the range 680 < m(gluino) < 880 GeV have been excluded for higgsino masses between 180 and 800 GeV. Assuming high tan(beta), gluino masses between 680 and 820 GeV were excluded for 180 < m(higgsino) < 740 GeV.