UC Santa Cruz

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).