Mesut Ünal, UT-Austin

"Search for vector-like quarks with the ATLAS experiment at the Large Hadron Collider"

Abstract: This thesis describes a search for new, heavy fermions using LHC data collected in 2015-2018 with the ATLAS detector. A search for single production of a vector-like top quark (VLT) decaying to a Z boson and a top quark in the dilepton final state is presented. This high statistics decay channel has high discovery potential thanks to the possibility of VLT mass reconstruction using the Z boson and top quark masses. The top quark decaying from a VLT is typically found in highly boosted, large-radius hadronic jets, which makes the identification challenging. To overcome this, novel top quark candidate identification techniques have been developed using the substructure information of variable-radius re-clustered jets. Variable-radius re-clustering uses large jets within a cone of momentum-dependent radius. The substructure information most sensitive to the top quark identification includes the mass, transverse momentum, and number of jet constituents. In this thesis, a detailed statistical analysis of the results of the search for VLT are presented. No deviation from the Standard Model is observed. The strongest to date observed and expected limits on the vector-like top mass as a function of coupling constant are set. In the case of VLT singlets with a coupling of kappa = 0.5, masses below 1810 GeV are excluded at the 95% confidence level.