Search for supersymmetric particles in final states with multi-leptons with the ATLAS detector

Abstract : The Large Hadron Collider (LHC), largest collider located at CERN (European Organization for Nuclear Research). produces proton-proton collisions collected by the ATLAS detector since 2009 at a collision center of mass energy up to 8 TeV before 2012 (Run1) and 13 TeV in 2015 and 2016 (Run2). Supersymmetry (SUSY) theory models, extension of the Standard Model (SM), link each boson (of integer spin) to a certain fermion (of half integer spin) as super-partner. New elementary particles are then introduced.After a brief presentation of the Standard Model and of the main SUSY models, the LHC and the ATLAS detector are described followed by a performance study on tau isolation. The main part of the document finally describes in details two searches for SUSY particles with the ATLAS detector and the obtained results.The first one is a search for direct stau production with final state of two opposite sign taus and multi-jets in 8 TeV pp collision and 20.1 fb-1 integrated luminosity. Multivariable analysis techniques are used to improve the sensitivity. No significant excess over the SM expectation was observed. Upper-limit was set on the cross-section of the signal models. For “direct stau” search, we excluded the signalpoint with m(LSP) = 0 GeV and m(stau) = 100 GeV.The second one is a search for squarks and gluinos strong production in final states with jets and two same-sign leptons or three leptons at 13 TeV and 13.2 fb-1 integrated luminosity. No significant excess over the SM expectation was observed. Upper-limit was set on the cross-section of all the models involved. Region with gluino mass up to 1.7 TeV have been excluded.
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Huan Ren. Search for supersymmetric particles in final states with multi-leptons with the ATLAS detector. High Energy Physics - Experiment [hep-ex]. AMU Aix Marseille Université, 2017. English. ⟨tel-02279517⟩

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