Search for compressed scalar top quark pairs in p-p collisions at LHC at $\sqrt{s} = 13 TeV$

Supersymmetry (SUSY) is one of the most appealing theories, which tries to explain some of the experimental and theoretical shortcomings of the Standard Model. It tries to solve the Hierarchy problem by introducing new particles (supersymmetric partners of the Standard Model particles). The quantum corrections due to these particles cancel the divergent contribution from the SM particles to the Higgs mass calculation. The top quark couples to the Higgs maximally due to its higher mass. Therefore, particularly, the supersymmetric partner of the top quark- scalar top quark(stop) is interesting for the searches to stabilize the Higgs mass. It is not easy to single out a model in supersymmetry due to the large number of free parameters. So, theorists have come up with some simplified models, which can be tested experimentally. This thesis is based on one of such models, where a stop decays to a top quark and a neutralino, where neutralino is assumed to be the Lightest Supersymmetric Particle (LSP). It is weakly interacting and stable. Hence, it does not leave any trace in the detector contributing to the missingtransverse energy (MET) in the event. This additional source of MET differentiates the signature of $t\overline{t}$ background from the signature of the signal $\tilde{t}\tilde{t}$ events. Some of the popular MET based analyses have been successful in excluding stop particles upto the mass of 900 GeV. But, the lower mass regions still have weaker exclusion limits. Especially,the region in the stop-LSP mass plane described by $M_{\tilde{t}} \approx M_{t} + M_{\tilde{X}^0_1}$, contains some model with lower stop masses, which have not been excluded yet due to the limitations of the currentMET based searches. By selecting only the events with a hard ISR (Initial State Radiation) jet, the MET of the entire system can be improved and also the lower mass models can be probed. For the events produced using the Monte Carlo simulations, expected upper limits at 95% confidence level were calculated for the integrated luminosity L = 30 $fb^{−1}$, 35.9 $fb^{−1}$ and 100 $fb^{−1}$ at $\sqrt{s} = 13 TeV$ of centre of mass energy. These limits exclude stop masses in the range 225 $M\tilde{t}$ < 450. Forthe lower masses of LSP, another technique based on the fermionic and bosonic nature of the t and $\tilde{t}$, respectively, is used to study the effect of the spin correlations between the decay products of the top or stop pair produced. A dileptonanalysis can be used to study $\Delta\phi$ (l1 , l2 ) distributions (l1and l2 are the two leptons) and calculating the limits using the events in$\Delta\phi$ (l1 , l2 ) bins, give betterexclusion potential for the models with $M\tilde{t}$ < 225 GeV compared to that given by the full hadronic ISR tagging analysis.