Benefits of Minimizing the Vertex Detector Material Budget at the FCC-ee

With the discovery of the Higgs boson, the Standard Model of particle physics is complete. Yet, many questions about the nature of particles remain unanswered by it. The FCC-ee is a proposed lepton collider that will provide high precision measurements of Standard Model particle properties, which can reveal the tiniest deviations from Standard Model predictions. The vertex detector, which lies closest to the interaction point in a detector, is responsible for the reconstruction of the vertex, which is the position where the particle interaction occurs. Precise location of these vertices is for example important for the identification of heavy jets as well as tau leptons, which propagate a small distance from the primary vertex and decay at a secondary vertex. To accomplish this, the vertex detector should minimize the amount of material added to the detector in order to reduce the multiple scattering that particles experience. One concept for a detector at the FCCee is the IDEA detector. This Bachelor thesis investigates the benefits of minimizing the material budget of the IDEA vertex detector. Such a detector is based on the curved sensor technology developed for ALICE ITS3, where wafer-scale DMAPS sensors with a thickness of 20 to 40 𝜇m can be bent to form detector layers. In order to study the vertex detector performance, the metrics of impact parameter resolution, flight distance resolution, and secondary vertex resolution are investigated. To this end, DELPHES is used to perform fast simulation of detector response to 𝑍 → 𝜇+𝜇− events, particle gun muons, and strange B meson decays. It was found that reducing the thickness of the three innermost vertex detector layers from 280 𝜇m to 30 𝜇m shows significant improvement in impact parameter resolution (the fit parameter 𝑏 that describes the multiple scattering contribution is reduced by about 30%), as well as in flight distance resolution (about 6% improvement) and secondary vertex resolution (about 20% improvement). Even only reducing the thickness of the first vertex detector layer, which is most easily achievable since it is small enough to be constructed with one single silicon wafer, already improves the secondary vertex resolution by about 6%. Minimizing the thickness of the eight vertex disc layers yields a small additional improvement in performance, especially in the forward region, but is less influential than reducing the thickness of the inner barrel layers.