UC Santa Barbara

This dissertation presents a search for τ ̃ pair production conducted with 137 fb−1 of data collected between 2016 and 2018 by the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC). Supersymmetry (SUSY) can provide an elegant solution to the Higgs mass hierarchy problem, and supersymmetric models with the τ ̃ lepton as the lightest supersymmetric particle can potentially explain the dark matter relic density via τ ̃ and lightest SUSY particle annihilation. In this search we look for purely left-handed and purely right-handed τ ̃ pair production, as well as the degenerate scenario where left- and right-handed pairs are produced together. A result with long- lived τ ̃s that give rise to hadronically decaying τ leptons is presented for the first time. The search is performed in the all-hadronic final state, and the background estimation uses data-driven and hybrid data-driven and simulation techniques. The observed yields are consistent with Standard Model predictions. Masses below 350 GeV and below 400 GeV are excluded at the 95% confidence level in the left-handed and degenerate scenario respectively.