Search for top squark pair production and decay in $t$ and $\tilde{\chi}^{0}_{1}$, with two leptons in the final state, at the ATLAS Experiment with LHC Run 2 data

The elementary particles, together with the strong, electromagnetic and weak interactions among them, are coherently described by the Standard Model (SM) of particle physics. The SM predictive power was tested several times and the discovery of the Higgs boson in 2012 at the Large Hadron Collider (LHC), performed by the ATLAS and CMS experiments, was a particular fundamental verification. However, there are several aspects in nature suggesting that SM cannot be a conclusive theory; it is, in fact, not able to explain, for example, the dark matter nature, the origin of the matter-anti-matter asymmetry in the universe, the neutrino masses and the huge difference between the Planck scale and the electroweak scale. All these issues need an extension of the Standard Model to be solved. Among all the various SM extensions (extra-dimensions, hidden sectors, extended Higgs sectors, etc.), Supersymmetry (SUSY) is one of the most promising, and it can also provide, in a $R-$parity conserving scenario, a natural candidate for the dark matter: the lightest neutralino. Futhermore, SUSY can lead to a possible couplings unification of the strong, electromagnetic and weak interactions. All this requires, however, the discovery of new particles, called "superpartners" of the Standard Model ones. In the work for my Ph.D. thesis, I focused my attention on the minimal supersymmetric extension of the Standard Model, known as Minimal Supersymmetric Standard Model (MSSM), analysing a $R-$parity conserving model where the superpartner of the top quark ($\tilde{t}_{1}$) decays into a top quark ($t$) and the lightest neutralino ($\tilde{\chi}^{0}_{1}$), with a branching ratio of 100%. The analysis was, in particular, developed to target the mass region $m_{\tilde{t}_{1}}\approx m_{t}+m_{\tilde{\chi}^{0}_{1}}$, usually called compressed region, characterised by a kinematic of the top quarks from $\tilde{t}_{1}\tilde{t}^{*}_{1}$ very similar to the one coming from $t\bar{t}$ process, making the distinction between the SUSY process under study and the SM process very challenging. The analysis uses the 36.1~fb$^{-1}$ of proton-proton collision data collected by the ATLAS experiment during 2015 and 2016 at $\sqrt{s}=13$~TeV, looking for final states with two opposite sign leptons (electrons or muons), at least three jets and large missing transverse energy.