Search for Higgs boson pair production in the b¯bτ +τ − final state with the ATLAS detector

Thesis

The self-interaction of the Higgs boson is a key prediction of the Standard Model (SM), and its experimental verification through Higgs boson pair (HH) production remains one of the most important goals in particle physics, thirteen years after the Higgs boson’s discovery.

This thesis presents the search for non-resonant HH production via gluon fusion (ggF) and vector boson fusion (VBF), in the final state with two b-jets and two τ -leptons. The analysis is based on the full ATLAS Run 2 dataset of proton-proton collisions, corresponding to an integrated luminosity of 140 fb−1 at a centre-of-mass energy of 13 TeV. An observed (expected) upper limit on the signal strength of µHH < 5.9 (3.3) times the SM prediction is set at the 95 % confidence level, ranking this analysis among the three most sensitive HH searches to date. The expected limit is improved by 15 %, corresponding to approximately 60 fb−1 of additional data in this statistically limited analysis. Moreover, the optimised analysis strategy enables the first independent measurements of the ggF and VBF HH production signal strengths. This leads to enhanced sensitivity to the Higgs boson self-coupling modifier κλ and the quartic coupling modifier κ2V (V = W, Z) with observed (expected) 95 % confidence intervals of κλ ∈ [−3.1, 9.0] ([−2.5, 9.3]) and κ2V ∈ [−0.5, 2.7] ([−0.2, 2.4]), corresponding to expected improvements of 11 % and 19 %, respectively. A dedicated extrapolation study conducted as part of this thesis demonstrates that the observation of HH production is within realistic reach at the High-Luminosity LHC (HL-LHC). In preparation for the HL-LHC, this thesis also investigates the Detector Safety System (DSS), focusing on alarm behaviour during upgrade and maintenance activities.

A detailed study reveals that alarms are significantly more frequent during these periods. To improve operational safety and diagnostics, a new tool - the Alarm Helper - was developed. It facilitates alarm follow-up and provides a structured database that will support future safety improvements during ATLAS operations.

Authors: Haslbeck, F

• DOI: 10.5287/ora-o58mnmzgm
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: detector safety system; LHC Run 2; Higgs boson pair production; high-luminosity LHC; particle physics; ATLAS detector
• Subjects: Physics; System safety