Date of Award

2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Physics & Astronomy

First Advisor

I. J. Kroll

Abstract

This thesis presents two searches for direct production of “electroweakinos”, the supersymmetric partner particles (“superpartners”) of the Standard Model photon, W, Z, and Higgs bosons. These searches were performed using 36 fb⁻¹ of √s = 13 TeV data collected by the ATLAS detector during Run 2 of the Large Hadron Collider (LHC). The first search focuses on a final state with three prompt leptons (electrons or muons) from on-shell W and Z bosons produced in decays of the electroweakinos. The second search focuses on compressed scenarios where the electroweakinos decay via off-shell W and Z bosons to pairs of leptons with small transverse momenta (pT). This second search allowed for the first exclusions of directly produced compressed Higgsinos at the LHC. In describing these searches, particular emphasis is placed on the estimation of backgrounds which mimic prompt leptons. This thesis also details the ATLAS electron identification algorithm, which distinguishes prompt electrons from backgrounds using a likelihood-based method. Significant improvements have been made to this algorithm during Run 2 including reoptimization of the selection criteria for new detector conditions, extension of the algorithm to a wider range of electron pT, and adaptation of the algorithm for the real-time selection of events. The corresponding measurements of the efficiencies for electron identification and other electron selection criteria are also described. Finally, readout electronics designed for upgrades to the ATLAS tracking system for use during the high-luminosity phase of the LHC are discussed. During this high-luminosity phase, an expected 3000 fb⁻¹ of data will be delivered to ATLAS. It is therefore essential to ensure that ATLAS will be able to efficiently operate and collect data despite the significant radiation damage it will receive. The expected performance of one readout chip (“AMAC”) is demonstrated via functionality and irradiation tests of several prototypes, and good performance is observed.

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