Search for squarks and gluinos with two same-sign or three leptons final states using 13 TeV proton-proton collision data with the ATLAS detector

Abstract

Supersymmetry (SUSY) is a compelling extension of Standard Model (SM) in particle physics, which introduces superpartners to SM particles by assuming a new symmetry relates fermionic and bosonic particles. These predicted SUSY particles can provide a natural solution to the hierarchy problem in SM. By assuming an ad hoc symmetry called R-parity, the lightest SUSY particle is stable which can be regarded as a suitable 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 observed baryon asymmetry or the neutrino masses. The Large Hadron Collider (LHC) offers a great opportunity to search for SUSY particles at TeV scale. This thesis presents a direct search for SUSY particles in events with two same-sign leptons or three leptons in the final states. The physics analysis targets at pair production of strongly-interacting squarks and gluinos, known as superpartners of quarks and gluons in SM. The dataset used in the analysis was collected from 2015 to 2018 by the ATLAS detector at LHC in proton-proton collisions at a centre-of-mass energy of √s = 13 TeV, corresponding to an integral luminosity of 139 fb−1. Multiple signal regions have been designed and optimized for targeting SUSY simplified models. A single control region is used to constrain the normalization of the WZ+jets background. With a sophisticated statistical analysis being performed in these regions, 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 previous results. The model-independent upper limits on the BSM events that may contribute to the analysis regions have also been obtained.