Search for long-lived charginos based on a disappearing track signature in pp collisions at $\sqrt{s}$ = 13 TeV with the full Run-2 ATLAS data

Supersymmetry (SUSY) is one of the most attractive concepts of the standard model extension. The lightest SUSY particle (LSP) is a good candidate for dark matter. The pure-wino or -higgsino LSP scenario is particularly motivated well by the relic density measurement and the pMSSM scan result. Under these scenarios, the masses of the lightest chargino and neutralino highly degenerate, and the chargino can have a long lifetime. This thesis presents the search results for long-lived charginos using a disappearing track comprising four innermost pixel layer hits in $pp$ collisions at $\sqrt{s}$ = 13 $\text{TeV}$ with the full Run-2 ATLAS data. In this analysis, the production of the $\tilde\chi^\pm_1$-$\tilde\chi^0_1$ or $\tilde\chi^+_1$-$\tilde\chi^-_1$ pair with an initial state radiation jet (EWK channel) and the production of a gluino pair with cascade decay into the chargino (strong channel) are considered as the signal production processes. The search is performed for the pure-wino and -higgsino LSP scenarios in the electroweak channel and the pure-wino LSP scenario in the strong channel. Unfortunately, no significant excess existed above the background predictions in both channels; hence, new exclusion limits at 95\% CL were calculated for each signal model. The observed limit in the EWK channel excluded chargino masses up to 660 GeV/$c^2$ on the theoretical prediction line for the pure-wino LSP scenario and 210 GeV/$c^2$ for the pure-higgsino LSP scenario. The maximum reach in the strong channel was 1.4 (1.8 TeV/$c^2$) and 2.1 (2.18 TeV/$c^2$) TeV/$c^2$ on the chargino and gluino masses, respectively, with $\tau_{\tilde\chi^\pm_1}$ = 0.2 ns (1.0 ns). The search results are compared to those of other analyses and discussed. Several ideas for improving sensitivities for future disappearing track searches are also introduced. The expected sensitivity using various track lengths is shown. Finally, assuming that the long-lived chargino is already discovered, the chargino mass reconstruction method and its accuracy are explained.