Search for gluinos in final states with jets and large missing transverse momentum using 36fb$^{-1}$ data observed in the ATLAS detector

Supersymmetry is one of well-motivated theoretical frameworks beyond the Standard Model ("SM") in the elementary particle physics. The supersymmetric models predict new particles accompanying all of the SM particles. It is promising that gluino is discovered at the LHC because of its relatively light mass and its large production cross-section in a proton-proton collision. In this thesis, gluino is searched for with the ATLAS detector in the proton-proton collision at the LHC in final states with multiple jets and large missing energy ($E^{miss}_{T}$) originating from undetected supersymmetric particles $\tilde{\chi}^{0}_{1}$ (the lightest neutralino). In order to improve the sensitivity especially for the high $\tilde{\chi}^{0}_{1}$ mass around 1 TeV, new techniques of quark/gluon separation and multivariate analysis are introduced. For a direct decay signal with a high gluino mass and a high $\tilde{\chi}^{0}_{1}$ mass, they make a gain in the background rejection power by factor 2 from the previous study. As a result of this search using 36.1fb$^{−1}$ data recorded in 2015 and 2016 at the center-of-mass energy $\sqrt{s}=13$ TeV, there is no significant excess indicating the gluino. Strongest exclusion limits on the gluino and $\tilde{\chi}^{0}_{1}$ masses are obtained for gluino direct and one-step decay models. Especially, the high $\tilde{\chi}^{0}_{1}$ mass region in the high gluino mass range is more effectively searched than the previous study. The $\tilde{\chi}^{0}_{1}$ mass is excluded up to 1 TeV in the gluino mass range of 1.50–1.80 TeV for the gluino direct decay, and excluded up to 0.85 TeV in the gluino mass range of 1.25–1.85 TeV for the gluino one-step decay at 95% confidence level.