Search for Supersymmetry with Tau Leptons, Muons, Missing Transverse Momentum and Jets with the ATLAS Experiment at the Large Hadron Collider

Abstract

Beyond the established Standard Model (SM), the search for new physics is among the most important ventures of modern physics. Supersymmetry (SUSY) is a natural candidate for an extension of the SM to account for unexplained phenomena. Therefore, the search for SUSY is an essential part in the physics program of collider experiments. With the ongoing exclusion of SUSY models, the scope of SUSY searches is more and more extended towards models predicting tau lepton final states. Tau leptons might be a unique probe for the existence of SUSY, but these SUSY models could still be veiled by the less striking tau signatures in the detector. This thesis presents a search for SUSY with tau final states at the ATLAS experiment at the LHC. It is among the first published SUSY searches with tau leptons in ATLAS.<br /> The standard tau lepton analyses search for hadronic tau decays and typically suffer from fake tau signatures of various origins. These contributions contaminate the data with unwanted backgrounds. In contrast to hadronically decaying tau leptons, muons have a very characteristic detector appearance. They show up with a clear signature that can be measured with excellent accuracy and high efficiency. The work presented in this thesis extends the searches for pure hadronic tau final states towards final states with muonic tau decays. It is shown that the muon signature helps to reduce background contributions and to probe the identification performance of hadronically decaying taus in the complex experimental environment of the ATLAS detector. In particular, the muon can be used to control fake tau contributions, which is of great benefit for an accurate background prediction.<br /> Based on these studies, SUSY is searched in final states with one muon, at least one hadronically decaying tau lepton, jets and missing transverse energy. The search is performed in proton-proton collisions at √s = 7 TeV corresponding to an integrated luminosity of 4.7 fb^-1. The considered SUSY model is Gauge Mediated Supersymmetry Breaking (GMSB) with the parameters N₅ = 3, sgn(µ) = +1, C_G = 1 and S = 250 TeV. Λ and tanβ are left as free parameters of the model. The event selection is optimized for maximal sensitivity to GMSB and leaves an expected SM background of 0.80± 0.28^{MC stat.}± 0.275^syst. events. The measurement of one event in data agrees well with the SM expectation and the non-observation of any deviation is used to set limits on the possible existence of GMSB models. GMSB models up to Λ = 45 TeV could be excluded with 95 percent confidence level. The limit is further tightened by the statistical combination with other SUSY searches with tau final states.