Supersymmetry (SUSY) is an extension of the Standard Model that predicts a boson
(fermion) partner for each fermion (boson) in the Standard Model. Weak-scale SUSY is
attractive for reasons like improving gauge coupling unification, reducing fine-tuning in
the Higgs sector and providing a dark matter candidate. This thesis presents a dedicated
search for direct production of new, colorless, weak-scale states with compressed mass
spectra in final states characterized by soft visible decay products. This analysis uses
pp collisions at √s = 13 TeV at the Large Hadron Collider and collected by the ATLAS
experiment during 2015 and 2016 corresponding to 36.1 fb−1 of integrated luminosity.
This analysis selects events with two soft electrons or muons, an intermediate amount of
missing transverse momentum (Emiss), and a hard jet. Backgrounds with two prompt T
leptons are estimated with Monte Carlo simulation, while reducible backgrounds are estimated with a mix of Monte Carlo and data-driven methods. Results are consistent with Standard Model expectations and used to put limits on compressed supersymmetric states. Limits are extended on compressed electroweak SUSY model for the first time since the Large Electron Positron Collider (LEP).