Search for single production of vector-like quarks decaying into a <em>Wb</em> final state at √s = 13 TeV with the ATLAS detector

Abstract

In this thesis, a search for single-production of vector-like quarks was performed using data collected during Run 2 of the Large Hadron Collider (LHC) by the ATLAS detector. The primary focus of this thesis was on the single production of vector-like quarks (<em>T/Y</em>) decaying into a <em>Wb</em> final state. This work can be divided into two major analyses, each targeting orthogonal phase spaces. <br /> The first analysis concentrated on the <em>T/Y</em> vector-like quarks decaying via a leptonic channel, where the <em>W</em>-boson decays into either an electron or a muon along with a neutrino. This analysis utilised <em>pp</em> collision data from 2015-16. The strategy involved identifying high transverse momentum (<em>p_T</em>) <em>b</em>-jets with significant angular separation from the leptons. This analysis also considered interference effects between the signal and background processes, leading to the development of novel statistical methods for calculating coupling limits. Ultimately, no significant excess over the Standard Model background was observed, and mass and coupling limits were set for different multiplet representations of the vector-like quarks. <br /> The second analysis explored the same final state through the hadronic decay mode, where the <em>W</em>-boson decays into a quark-anti-quark pair. This analysis, which is first of its kind, uses ATLAS data collected from 2015 to 2018. Due to the high masses of the <em>T/Y</em> quarks under investigation, their decays resulted in Lorentz-boosted <em>W</em>-bosons. The reconstruction of the <em>W</em>-jet was accomplished using advanced jet-taggers. To account for the substantial mismodelling in the background simulation, data-driven ABCD methods were introduced to accurately estimate the multijet background. Like the previous analysis, no significant excess was observed, and mass and mass-coupling limits were established for the <em>T/Y</em> quarks, thus introducing new physics constraints. <br /> This thesis also underscores the development of novel statistical methods to handle interference effects and presents an improved ABCD method, which could benefit future hadronic searches. By tackling both leptonic and hadronic decay modes, this work explores the full phase space for <em>T/Y → Wb</em> decays during Run 2 of the LHC. The research lays the foundation for further investigations by the ATLAS collaboration and underscores the potential of advanced jet-taggers in exploring hadronic final states for future LHC runs.