Search for scalar top-quark pair-production of compressed SUSY scenarios in the final state involving one lepton, jets, and missing transverse energy in $pp$ collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector

In 2012, the Higgs boson was discovered by the Large Hadron Collider (LHC) experiment at CERN. However, to derive the observed Higgs mass (125 GeV) in the Standard Model (SM), fine tuning between the bare Higgs mass and the radiative correction required. The SM has another problem, which is the absence of the particles constituting the dark matter (DM) indicated by the cosmological observation. One of the candidates of the theory which can solve these problems is the Supersymmetry (SUSY). If the scalar top quark (stop, $\tilde{t}$), which is the superpartner of the SM top quark, exists and has the mass below 1 TeV, the level of fine tuning can be significantly reduced because the radiative correction of the top quark loop can be canceled by the radiative correction of the stop loop. In addition, the neutralino ($\tilde{\chi}^{0}_{1}$), which is the neutral lightest supersymmetric particle (LSP) can become a candidate of the DM. The LHC experiment searched for the stop pair production in a $pp \rightarrow \tilde{t}_{1}\tilde{t}_{1} \rightarrow t\tilde{\chi}^{0}_{1} t\tilde{\chi}^{0}_{1}$ process and set a 1 TeV mass limit by exploiting the highest center-of-mass energy of the $pp$ collisions. Unfortunately, the evidence of the stop pair production was not obtained. The direct searches for the stop pair production in the phase space challenging for the experiments is of interest. An example is the phase space, where two or three of stop, chargino ($\tilde{\chi}^{\pm}_{1}$), and neutralino have similar masses. Here a search is presented for the stop pair production in a $pp \rightarrow \tilde{t}_{1}\tilde{t}_{1} \rightarrow b\tilde{\chi}^{\pm}_{1} b\tilde{\chi}^{\pm}_{1} \rightarrow bW^{\pm}\tilde{\chi}^{0}_{1} bW^{\pm}\tilde{\chi}^{0}_{1}$ process with the 36.1 ${\rm fb}^{-1}$ data of $\sqrt{s} = 13$ TeV $pp$ collisions obtained by the LHC-ATLAS experiment from 2015 to the end of 2016. The search focuses on two theoretical scenarios, which have the compressed mass spectrum between the SUSY particles (sparticles) and can be searched by the final state involving one lepton, jets, and missing transverse energy ($E^{\rm miss}_{\rm T}$): I. the higgsino LSP scenario with small mass difference between the chargino and the neutralino and II. the bino LSP scenario with small mass difference between the stop and the chargino. In case of the higgsino LSP scenario, the signal region (SR), which has a high $E^{\rm miss}_{\rm T}$ and an initial state radiation with a high momentum, was set and optimized. The dominant backgrounds ($t\bar{t}$ and $W$+${\rm jets}$) remaining after the signal event selection were estimated by using a semi-data driven method called the control region (CR) techniques to reduce the systematic uncertainties. The QCD/multi-jets background including a fake leptons was estimated to be negligible. No significant excess above SM expectation was observed in the SR. This result was reinterpreted to determinate the exclusion limit. The higgsino LSP scenario was excluded the stop mass up to 415 GeV. In case of the bino LSP scenario, the SR is introduced with rejecting jets tagged as originating from bottom quarks. The dominant background ($W$+${\rm jets}$) was estimated by using the CR techniques. No significant excess above the SM expectation was observed in the SR. The bino LSP scenario was excluded in a part of the phase space with the stop mass up to 850 GeV. The coverage in the phase space has been significantly extended by setting and optimizing the individual SRs dedicated for the two scenarios and by precisely estimating the background. I contributed to the strongly constrain for SUSY, in particular the stop pair production, by searching these scenario having the compressed mass difference between sparticles.