SEARCHES FOR SUPERSYMMETRY AT THE HIGH-LUMINOSITY LHC


Thesis Type: Post Graduate

Institution Of The Thesis: Orta Doğu Teknik Üniversitesi, Faculty of Arts and Sciences, Department of Physics, Turkey

Approval Date: 2018

Student: GÖKÇENUR YEŞİLYURT

Consultant: MELTEM SERİN

Abstract:

High Luminosity LHC (HL-LHC) project is an upgraded version of the LHC that enables to explore beyond the Standard Model theories and is expected to exceed the limits of currently conducted experiments. Supersymmetry is one of the commonly used and well-known beyond the Standard Model theories proposing a symmetry with brand-new particles and by doing so, bringing solutions to the unsolved zones that the Standard Model is not available to reach. In this thesis, by considering the high luminosity and the center of mass energy that are going to be achieved in scope of the HL-LHC project, single lepton and di-lepton final state channels are examined at $sqrt{s}$=14 TeV. Three pile-up with two luminosity cases such as, for No pile-up and 50 pile-up, 300 $fb^{-1}$ and for 140 pile-up 3000 $fb^{-1}$ luminosity values are used. For the Standard Model background samples ($tbar{t}$+jets, Boson+jets, Single t+jets and Di-boson) including with the signal sample (STC8, sTau-coannihilation) Pythia is used for simulation of them. Detector atmosphere is simulated via Delphes to insert the effects in all of the samples. In order to make an estimation in the most efficient cut flows for the Supersymmetry studies that are going to be conducted at HL-LHC and to develop a fundamental analysis technique, two different cut options for $E_T^{miss}$ and $H_T$ are studied. Both of these variables are preferably used in most Supersymmetry searches and preferred to be studied for both of the final state channels in this thesis also. Besides these variables, the new topological variables, $M_{T2}^W$ and $topness$ are also used in single lepton final state analysis. The positive impact in suppressing the background events by applying cuts starting from $E_T^{miss}$ has been observed especially for the di-lepton channel yet the crucial gain is obtained by inserting the $M_{T2}^W$ and $topness$ variables in single lepton channel. The discovery threshold has been exceeded for all of the options in scope of the expected single lepton final state, whereas more number of events are needed to increase the statics for the di-lepton final state in order to obtain a significance over $5sigma$ for all of the cut options. The effectiveness of the topological variables in significance are almost the same at all pile-up scenarios. The highest significance value for single lepton is attained for No pile-up case, even if this case is accepted as an optimistic alternative, with a higher cut in $E_T^{miss}$. However for di-lepton case, 140 pile-up with lower $H_T$ cut leads the maximum probability.