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Supersymmetry (SUSY) is a compelling extension of Standard Model (SM) in particle physics, by assuming a new symmetry relates fermionic and bosonic particles. The Minimal Supersymmetric Standard Model (MSSM) introduces a supersymmetric partner particle for each particle in SM by varying its spin by a half of unit. These SUSY particles predicted by MSSM can provide a natural solution to many open questions in SM. The hierarchy problem can be resolved with the presence of superpartners for SM particles by automatically cancelling the divergent loop corrections on Higgs boson squared mass calculation. By assuming an ad hoc symmetry called R-parity, the lightest SUSY particle is stable which can be regarded as a promising candidate for dark matter. If R-parity is not conserved, the violation on baryon number or lepton number is allowed, which can explain new physics phenomena beyond SM (BSM) such as the baryon asymmetry or neutrino masses.

The world's largest and most powerful particle collider, the Large Hadron Collider (LHC) at CERN, offers a great opportunity to search for SUSY particles at TeV scale under high energy proton-proton collisions. In this seminar, I am going to present a recent search for direct production of SUSY particles in events with two same-sign leptons or three leptons in the final states. This search targets at pair production of strongly-interacting squarks and gluinos, known as superpartners of quarks and gluons in SM. The analysed dataset was collected from 2015 to 2018 by the ATLAS detector at LHC in proton-proton collisions at a centre-of-mass energy of 13 TeV. No significant excess has been observed over SM predictions. The results are interpreted in the context of different R-parity conserving and R-parity violating SUSY scenarios. Stringent exclusion limits at 95% confidence level are placed on masses of the superpartners involved in the considered scenarios, which have significantly improved results from previous searches. The model-independent upper limits on the BSM events that may contribute to the analysis regions are also computed which provide good sensitivity on general BSM signals.