Phenomenology of $A^{0}\to Z^{(*)}h^{0}$ in 2-Higgs Doublet Models

Abstract

This thesis investigates the search for the CP-odd scalar $A^{0}$ in the Large Hadron Collider (LHC) in 2-Higgs doublet models (2HDMs), focusing on $A^{0}\rightarrow h^{0} Z$ in the inverted hierarchy (IH) scenario, where the CP-even scalar ($h^{0}$) has a mass below 125 GeV. This decay has been investigated in three different projects. The first calculated $ \sigma (gg \to A^{0})\times BR(A^{0}\to h^{0}Z^{*})\times BR(h^{0}\rightarrow b\bar{b}, \tau \tau )$ across all four types of 2HDMs with Natural Flavour Conservation (NFC), specifically in the IH scenario. The current search optimises selection criteria for cases where the Z boson is on-shell. However, no constraints have been established for cases involving an off-shell Z boson ($Z^{*}$), which occurs when $m_{A^{0}}- m_{h^{0}}< m_{Z}$. This off-shell decay mode can have a significant branching ratio, particularly in 2HDMs Type I. In the second project, in the LHC, the production and decay of the CP-odd scalar $A^{0}$ occurred via $gg\to A^0$ (through one-loop triangle diagrams) and $A^0\to h^0 Z^*$ (with $m_{h^0} =125$ GeV or $m_{h^0} < 125$ GeV, with $Z$ off-shell). However, this method overlooks additional contributing channels, such as $gg\to Z^*\to h^0Z^*$ (through one-loop triangle diagrams) and $gg\to h^0Z^*$ (through one-loop box diagrams). We calculated simultaneously all contributions, including the interference and decay products of $h^{0}$ and $Z^{*}$. Significant differences were found between these methods, particularly in differential analysis, where deviations can be up to several tens of percent, while integrated rates remain within 10\%. Then in the last project, a simulation and analysis of the signal and main two background processes, Drell–Yan (DY) and top-quark pair production, were performed for invariant mass cuts on di-leptons ranging from 20 GeV to 50 GeV. This mass cut will capture $Z^{*}$ peak, which has not been done before experimentally. In the ATLAS and CMS analyses, the standard mass window of 70–110 GeV was examined to ensure that the Z boson was on-shell.