Long-Lived Particles from exotic Higgs decays at the FCC-ee

Abstract: This thesis presents the first sensitivity analysis for exotic Higgs boson decays at the electron-positron stage of the Future Circular Collider (FCC-ee) within the FCCAnalyses framework. The project studies the production of a Higgs boson in association with a Z boson in electron-positron collisions at a center-of-mass energy of 240 GeV, with the Higgs boson decaying into two long-lived scalars and the Z boson decaying leptonically. A hadronic signature is considered, with the long-lived scalars decaying to a bottom anti-bottom quark pair. Simulations are set up in the FCCAnalyses framework, using MadGraph and Pythia for the event generation and Delphes for the detector simulation. Displaced vertices from the long-lived particle decays are reconstructed based on the FCCAnalyses version of the secondary vertex finding algorithm LCFIPlus, with extended functionality such as custom track selection. A final event selection requiring the presence of a Z boson and at least two displaced vertices is seen to efficiently remove the Standard Model background, while leaving three or more expected events, including statistical uncertainties, for the majority of the considered parameter space with scalar masses of 20 and 60 GeV and mixing angles of 10-5, 10-6 and 10-7. The results suggest that the FCC-ee will be sensitive to long-lived scalars with decay lengths of order 1 mm to 10 m, with the peak sensitivity for decay lengths around 0.3 m.