Janis McKenna

The so far unresolved question of the nature of dark matter has led to the development of a multitude of particle physics models that all meet current experimentaland theoretical constraints, but differ greatly in the experimental signatures theypredict. One idea that has been gaining popularity is that there could be a wholefamily of dark matter particles, forming a so called ’dark sector’ that interacts onlyvery feebly with regular matter. In this thesis we investigate a hypothetical stronglyinteracting dark sector that confines at the GeV scale, which results in the production of a shower of dark sector mesons, including a long-lived neutral particle. Thiswould result in a displaced vertex signature at e⁺e⁻ colliders such as the Belle IIexperiment. We provide a brief overview of current evidence for Dark Matter andexisting constraints to motivate the choice of a QCD–like dark sector with a darkphoton mediator, as well as a detailed description of the Belle II detector which hasan excellent sensitivity to displaced vertex signatures. After briefly summarizingthe model, we describe the general search strategy, examine possible sources ofbackground, and test the feasibility of a zero-background analysis. We find that,while not all signatures can be investigated in the zero-background case, the modeloffers a large parameter space that can be well investigated at Belle II with currently available data.

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