Subject Area
Physics
Abstract
The Higgs boson self-coupling (λ) is a fundamental parameter of the Standard Model that sets the shape of the Higgs potential and probes the mechanism of electroweak symmetry breaking. This coupling can be directly measured through Higgs boson pair production (HH), which has a predicted production rate roughly 1000 times smaller than single-Higgs production at the LHC.
A combined search for non-resonant Higgs boson pair production in final states with multiple light leptons (e,µ), hadronically decaying τ leptons, and photons is presented, using 140 fb−1 of 13 TeV proton-proton collision data collected by the ATLAS detector during Run 2 of the LHC (2015-2018). Nine final states are analyzed, targeting HH decays to WW, ZZ, ττ, bb, and γγ in combinations that produce multiple leptons. This is the first ATLAS analysis to explore these multilepton topologies for Higgs pair production. The channels complement existing ATLAS searches in the bbγγ, bbττ, bbbb, and bbℓℓ final states.
The multilepton analysis alone excludes Higgs boson pair production above 17 times the Standard Model prediction (11 expected) at 95% confidence level. Combined with other Run 2 ATLAS searches, the observed (expected) limit reaches 2.9 (2.4) times the SM prediction. The observed Higgs self-coupling κλ is constrained to [−1.2, 7.2], and the HHV V coupling κ2V is constrained to [0.6, 1.5] at 95% confidence level, both consistent with Standard Model predictions. These are the most stringent constraints on HH production to date.
High pileup degrades lepton isolation performance by adding energy deposits in the ATLAS calorimeters, reducing the ability to distinguish prompt leptons from jets. A pileup correction method for particle-flow-based electron isolation is presented that accounts for the pseudorapidity dependence of pileup density in the detector. This method improves electron isolation efficiency in high-pileup environments and establishes an approach for future LHC runs.
Projections for the High-Luminosity LHC are presented for the multilepton channels, extending the Run 2 analysis framework to 3000 fb−1 under four systematic uncertainty scenarios. The multilepton channels are projected to exclude HH production at 2.1 times the SM prediction in the baseline scenario. Combined ATLAS and CMS projections report that the HH signal strength could be measured to approximately 30% precision and κλ constrained to ±0.4 at 68% confidence level under optimistic scenarios. These projections were a major input to the 2026 European Strategy Update for Particle Physics.
Degree Date
Winter 5-5-2025
Document Type
Thesis
Degree Name
Ph.D.
Department
Physics
Advisor
Allison Deiana
Number of Pages
200
Format
Creative Commons License
This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License
Recommended Citation
Thornberry, Reagan E., "A New Avenue for Higgs Boson Self-Coupling Measurements: Combined Searches for Higgs Pairs in Multilepton Final States with the ATLAS Experiment" (2025). Physics Theses and Dissertations. 22.
https://scholar.smu.edu/hum_sci_physics_etds/22
