Abstract
This thesis presents the measurement of the CP-properties of the Yukawa coupling between the Higgs boson and τ−lepton. The measurement uses the proton–proton collision data collected during 2015 to 2018 with the ATLAS detector at the Large Hadron Collider. Totally 139 fb−1 proton–proton collision data is collected at a center-of-mass energy of √s = 13 TeV. This study investigates the CP-properties with CP-sensitive observables defined by the visible decay products of τ−leptons. CP-violating interactions between the Higgs boson and τ−lepton are described by the CP-mixing angle Ï•τ . The expected measured value of Ï•τ is 0â—¦±28â—¦ at 68% confidence level, and 0â—¦+75â—¦−70â—¦ at 95.5% confidence level at the Standard Model simulated dataset. The observed value of Ï•τ is 9â—¦±16â—¦ at 68% confidence level, and 9â—¦±34â—¦ at 95.5% confidence level. The pure CP-odd hypothesis is disfavoured at 3.4 standard deviation. The observation is consistent with the Standard Model expectations.
This thesis also presents the studies of pile-up jet tagging. The pile-up jet tagging algorithm is based on K-Nearest Neighbor method. It is trained with the data collected during 2015 to 2018 with the ATLAS detector. A new pile-up jet tagging algorithm based on neural network is developed. It provides improvement to the background rejection at all working points.
In this thesis, a new CP-sensitive observable is developed using neural network approach. The trained observable excludes the pure CP-odd hypothesis at higher confidence level in a simplified sample compared with the previously designed observable. These studies will contribute to further precision measurements of the CP-properties.