The Development of the ATLAS SCT as a Luminometer and the Search for SUSY Decays with Compressed Mass Spectra

The formulation of the Standard Model of particle physics (SM) is one of the greatest scientific achievements of the 20th century. It is, however, incomplete (for example, it lacks a dark matter candidate) as well as the fact that the hierarchy problem violates naturalness arguments. This has motivated the construction of particle accelerators to probe fundamental particles at increasingly high center-of-mass energies and luminosities, the LHC at CERN being the latest to continue this legacy. This thesis covers both the enhancement of luminosity measurements of pp collisions at ATLAS, underpinning the accuracy of all measurements made by the detector, and a search for one of the most theoretically viable extensions to the SM: supersymmetry. ATLAS uses mainly event-counting algorithms to measure luminosity, which break down at higher luminosities. If the ATLAS SemiConductor Tracker (SCT) can be employed as a luminometer using hit-counting algorithms, this issue may be mitigated. It is established here that the SCT can feasibly operate as a luminometer when recording two-or-more strip clusters with the standard binary readout mode (01X). Thus, the SCT can measure the luminosity with an accuracy within 10% of two of ATLAS's existing luminometers: LUCID and TileCal. The discovery of the supersymmetric top (stop) would be fundamental for solving the hierarchy problem. An analysis of an experimentally challenging region of phase space, where stop decays have a compressed mass spectrum, complements the ATLAS one-lepton stop search using 13 TeV pp collisions at 139 fb−1 is presented. The aMT2 kinematic variable, designed to give a lower limit on pair-produced particle masses, is found to be effective at differentiating SUSY decays from the SM background, when used as an upper bound. No significant excess was observed above the Standard Model background and limits at 95% confidence level are set. Stop quarks are found to be excluded up to 500 GeV and mass splittings between the stop and the neutralino are found to be excluded up to 130 GeV, complimenting the exclusion limits found by other ATLAS searches for stop decays with one-lepton final states.