Supersymmetry parameter determination at the International Linear Collider

In this thesis, the prospects for determining supersymmetry parameters from observations of supersymmetric particles at the International Linear Collider (ILC) are investigated. Supersymmetry (SUSY) has been proposed in order to solve gaps in the Standard Model of particle physics, among them the hierarchy problem and the unexplained dark matter content in the universe. There are many free parameters in supersymmetry, whose values give rise to the properties of the supersymmetric particles which can be searched for by experiments. While the Large Hadron Collider (LHC) has successfully excluded many possible scenarios, regions of parameter space remain unexplored, especially where the supersymmetric particles decay with little visible energy. For example, models with light higgsinos or scalar tau coannihilation can escape detection at the LHC. These kinds of scenarios could be discovered or excluded by the proposed ILC, which would collide polarised electron and positron beams at the centre-of-mass energy of 500 GeV. The advantages of this machine over the LHC, namely its the clean experimental environment and triggerless detector operation, would allow for the discovery of almost any particle within its kinematic reach. If supersymmetric discoveries were made at the ILC, it would be possible to measure the properties of the SUSY particles very precisely. These measurements enable determining some of the underlying SUSY parameters via fitting the parameters to the SUSY observations. In this thesis, a 10-parameter or 13-parameter phenomenological Minimal Supersymmetric Standard Model and high-scale 4-6-parameter models (CMSSM, NUHM1 and NUHM2) are fitted to sets of possible observations from the ILC. Two types of scenarios are considered: light higgsinos motivated by naturalness, and scalar tau coannihilation motivated by the dark matter relic density. It is shown that the precision measurements of the SUSY and Higgs sectors allow for determining some of the SUSY parameters. Additionally, strong predictions for unobserved heavy particle masses can be made, leading to guidance on future high-energy particle colliders. Furthermore, it is possible under certain circumstances to check whether the observed particles explain the dark matter relic density. It is shown that the permille or percent-level measurements from the International Large Detector are crucial for making these predictions. Additionally, the determined parameters in the weak scale fits are evolved to the GUT scale to test the gaugino mass unification hypothesis. The results give a strong argument for building an electron-positron collider to close the gaps in the LHC searches or to study any particles that the latter finds.