Reconstruction of leptonic physics objects at future e⁺e^- Higgs factory

Since the discovery of the Higgs boson in 2012 by the experiments at the Large Hadron Collider (LHC), precise measurement of Higgs boson has become the challenge in high energy physics experiments. Many electron-positron Higgs factories with improved accuracy on the Higgs total width measurements have been proposed, including the International Linear Collider (ILC), the Circular Electron Positron Collider (CEPC), the Future Circular Collider e⁺e^- (FCCee). In order to achieve the precision estimated to percent or sub-percent levels, the use of Particle Flow Algorithm (PFA) has become the paradigm of detector design for the high energy frontier. The key idea is to reconstruct every final state particle in the most suited sub-detectors, and reconstruct all the physics objects on top of the final state particles. The PFA oriented detectors have high efficiency in reconstructing physics objects such as leptons, jets, and missing energy. In this thesis, a PFA based lepton identification (Lepton Identification for Calorimeter with High granularity (LICH) has been developed for detectors with high granularity calorimeters. Using the conceptual detector geometry for the CEPC, and samples of single charged particles with energy larger than 2 GeV, LICH identifies electrons or muons with efficiencies higher than 99.5% and controls the mis-identification rate of hadron to muons or electrons to better than 1% or 0.5% respectively. Reducing the calorimeter granularity by 1 or 2 orders of magnitude, the lepton identification performance is stable for particles with E > 2 GeV. Applied to fully simulated eeH or μμH events at √(s) = 250 GeV, the lepton identification performance is consistent with the single particle case: the efficiency of identifying all the high energy leptons in an event ranges between 95.5% and 98.5%. The τ-decay products have low multiplicity and in high energy colliders are tightly collimated and have low multiplicity, providing excellent signatures to probe. In this thesis, the H → ττ channel is analyzed in different Z decay modes with SM background taken into account. The combined final accuracy of σ x Br(H → ττ) is expected to be 0.89%. (author)