Phenomenology of A0 → Z(∗)h0 in 2-Higgs Doublet Models

Abstract

This thesis investigates the search for the CP-odd scalar A0 in the Large Hadron Collider (LHC) in 2-Higgs doublet models (2HDMs), focusing on A0 → h0Z in the inverted hierarchy (IH) scenario, where the CP-even scalar (h0) has a mass below 125 GeV. This decay has been investigated in three different projects. The first calculated σ(gg → A0) × BR(A0 → h0Z∗) × BR(h0 → bb￣ , ττ) 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 mA0 − mh0 < mZ. 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 A0 occurred via gg → A0 (through one-loop triangle diagrams) and A0 → h0Z∗ (with mh0 = 125 GeV or mh0 < 125 GeV, with Z off-shell). However, this method overlooks additional contributing channels, such as gg → Z∗ → h0Z∗ (through one-loop triangle diagrams) and gg → h0Z∗ (through one-loop box diagrams). We calculated simultaneously all contributions, including the interference and decay products of h0 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.

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Identifiers

ORCID for Stefano Moretti: orcid.org/0000-0002-8601-7246

ORCID for Andrew Akeroyd: orcid.org/0000-0001-7742-7035

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