Investigation of non-resonant HH production and couplings in the bbbb final state using the ATLAS detector

Thesis

The full Run 2 ATLAS HH → bbbb analysis targeted the simultaneous non-resonant production of two Higgs bosons (HH) in the bbbb final state. This analysis utilised 126 fb^-1 of 13 TeV proton-proton collision data from the ATLAS Experiment on CERN’s Large Hadron Collider. The increase in data, alongside a program of intensive research and development that I played a significant role in, produced a comprehensive set of important physics results that showed a marked improvement on predecessor ATLAS HH → bbbb analyses. An observed (expected) upper limit on HH production via Gluon-Gluon Fusion (ggF) and Vector Boson Fusion (VBF) was set at 5.4 (8.1) times the SM prediction at the 95% confidence level (CL). Constraints on the coupling modifiers κ_λ , κ_V and κ_2V (defined as κ = c/c_SM ) were set using two different statistical methods. Performing a comparison of the theoretical cross-section to the upper limit on HH production, κ_λ was constrained to −3.9 < κ_λ < 11.1 (−4.6 < κ_λ < 10.8) and κ_2V to −0.03 < κ_2V < 2.11 (−0.05 < κ_2V < 2.12) at the 95% CL. Performing a direct measurement of the log likelihood ratio, κ_λ was constrained to −3.5 < κ_λ < 11.3 (−5.4 < κ_λ < 11.4) and κ_2V to 0.0 < κ_2V < 2.1 (−0.1 < κ_2V < 2.1) at 2σ. Two-dimensional limits were also set on a range of values in κ_2V-κ_λ and κ_2V-κ_V space at the 95% CL, the former being the first of its kind.

Authors: Grundy, JC

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: bottom quark; HH; ATLAS; DiHiggs
• Subjects: Particle Physics