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Abstract

In this thesis, we follow two approaches to explore the space of possible models of physics beyond the Standard Model, long-lived particle searches and effective field theories, following the papers I published during my Ph. D. studies. We explore the potential of long-lived particle searches at the e+ e− colliders Belle II and ILC, comparing their sensitivities to different other detectors and search methods. We find that these detectors have a strong sensitivity to these searches. We also explore the possibilities of adding far detectors to improve their sensitivities and find that realistic far detectors do not offer a great improvement in sensitivity to the Belle II and ILC detectors. We also compare the different search strategies of missing energy and displaced vertex searches for an axion-like particle model at Belle II, finding that both have the potential to explore new parameter space. While the missing energy search has a broader reach in mass, the displaced search can reach smaller couplings for heavy masses. We use the effective field theory approach to explore the flavour structure of UV physics by fitting SMEFT Wilson coefficients to top and bottom observables. In both approaches, we focus on observables from flavour physics, specifically B meson decays.