Jigsaws Falling into Place: Advances in Event Reconstruction and Electroweak Supersymmetry at ATLAS

Searches for beyond the Standard Model (BSM) physics are difficult to undertake, especially so at hadron colliders where the true energy of the collisions cannot be determined. An industry of analysis techniques and variables has been developed to more clearly separate BSM processes of interest, the archetype being supersymmetric (SUSY) particle decays, from Standard Model backgrounds. Recursive Jigsaw Reconstruction (RJR) is a technique which attempts to find the centre-of-mass reference frame of particle decays, solving for combinatoric and kinematic ambiguities by defining decay trees. This work describes the application of this technique to the electroweak production and decay of SUSY particles, using data collected by the ATLAS experiment at the Large Hadron Collider. A global likelihood analysis using the GAMBIT framework is also presented, placing the analysis in context with other results in high energy physics. Additionally, studies in analysis development using RJR are outlined, with the focus being on addressing combinatoric ambiguities in top quark processes. The development of electron identification efficiency at low energies at ATLAS is also presented in this work. While collision energies and intensities increase the requirement for low energy object reconstruction and identification remains a key factor in both Standard Model and BSM analyses. The results presented in this work allowed for the lower bound on electron energy to be reduced from 7 GeV to 4.5 GeV.