The discovery of the Higgs boson (H) by the ATLAS and CMS collaborations in 2012 completed the Standard Model (SM) of particle physics, providing the best description of fundamental interactions. Since then, several Higgs properties, such as mass, spin, production cross-section, and couplings to fermions and bosons, have been measured. However, its self-coupling, λ_HHH, remains largely unconstrained. This parameter is crucial for understanding the Higgs potential and the mechanism of electroweak symmetry breaking (EWSB), responsible for generating elementary particle masses. Higgs boson pair (HH) production offers a direct way to probe this self-coupling, but it remains unobserved due to its low cross-section. At the Large Hadron Collider (LHC), HH production occurs mainly via gluon-gluon fusion (ggF) and vector-boson fusion (VBF), both involving the trilinear Higgs self-interaction. Any deviation from the SM prediction for λ_HHH, expressed through the coupling modifier κ_λ = λ_HHH/λ_SM, could indicate new physics beyond the SM (BSM). This thesis presents the search for HH production in the bbγγ final state, using data from Run-II (2015–2018, √s = 13 TeV, 140 fb⁻¹) and Run-III (2022–2023, √s = 13.6 TeV, 59 fb⁻¹) collected with the ATLAS detector at the LHC. Despite its small branching ratio (0.26%), the b̄bγγ channel benefits from excellent diphoton invariant mass resolution (~2%), a smooth background, and a high H → b̄b branching fraction (59%), making it a key channel for HH searches. The SM predicts only 12 HH → b̄bγγ events in the Run-II dataset, suppressed by orders of magnitude compared to di-photon and single-Higgs backgrounds. The analysis focuses on selecting good H → bb and H → γγ candidates to optimize the signal-to-background ratio. Events are categorized to retain sensitivity to the HH signal strength and variations in κ_λ. HH production would appear as a resonant contribution in the di-photon invariant mass (m_γγ) spectrum, overlaid on the narrow single-Higgs peak and smoothly falling continuum background. The invariant mass of the two b-jets (m_bb) is the most discriminating variable in the Boosted Decision Tree (BDT) used for signal-background separation. While the H → γγ component is reconstructed with a few percent precision, the H → bb component suffers from a 15% invariant mass resolution. A key aspect of this thesis is the development of the Kinematic Fit Tool, which, assuming transverse-plane balance between H → bb and H → γγ, exploits the excellent H → γγ reconstruction to improve the H → bb resolution through event-level likelihood maximization. The final statistical results are obtained by applying the full analysis workflow to the bbγγ channel, incorporating b-jet corrections from the Kinematic Fit technique.
Montereali, F. (2025). Search for Higgs boson pair production in the two photons plus two bottom quarks final state using partial Run-3 proton-proton collision data with the ATLAS detector.
Search for Higgs boson pair production in the two photons plus two bottom quarks final state using partial Run-3 proton-proton collision data with the ATLAS detector
Federico Montereali
2025-03-21
Abstract
The discovery of the Higgs boson (H) by the ATLAS and CMS collaborations in 2012 completed the Standard Model (SM) of particle physics, providing the best description of fundamental interactions. Since then, several Higgs properties, such as mass, spin, production cross-section, and couplings to fermions and bosons, have been measured. However, its self-coupling, λ_HHH, remains largely unconstrained. This parameter is crucial for understanding the Higgs potential and the mechanism of electroweak symmetry breaking (EWSB), responsible for generating elementary particle masses. Higgs boson pair (HH) production offers a direct way to probe this self-coupling, but it remains unobserved due to its low cross-section. At the Large Hadron Collider (LHC), HH production occurs mainly via gluon-gluon fusion (ggF) and vector-boson fusion (VBF), both involving the trilinear Higgs self-interaction. Any deviation from the SM prediction for λ_HHH, expressed through the coupling modifier κ_λ = λ_HHH/λ_SM, could indicate new physics beyond the SM (BSM). This thesis presents the search for HH production in the bbγγ final state, using data from Run-II (2015–2018, √s = 13 TeV, 140 fb⁻¹) and Run-III (2022–2023, √s = 13.6 TeV, 59 fb⁻¹) collected with the ATLAS detector at the LHC. Despite its small branching ratio (0.26%), the b̄bγγ channel benefits from excellent diphoton invariant mass resolution (~2%), a smooth background, and a high H → b̄b branching fraction (59%), making it a key channel for HH searches. The SM predicts only 12 HH → b̄bγγ events in the Run-II dataset, suppressed by orders of magnitude compared to di-photon and single-Higgs backgrounds. The analysis focuses on selecting good H → bb and H → γγ candidates to optimize the signal-to-background ratio. Events are categorized to retain sensitivity to the HH signal strength and variations in κ_λ. HH production would appear as a resonant contribution in the di-photon invariant mass (m_γγ) spectrum, overlaid on the narrow single-Higgs peak and smoothly falling continuum background. The invariant mass of the two b-jets (m_bb) is the most discriminating variable in the Boosted Decision Tree (BDT) used for signal-background separation. While the H → γγ component is reconstructed with a few percent precision, the H → bb component suffers from a 15% invariant mass resolution. A key aspect of this thesis is the development of the Kinematic Fit Tool, which, assuming transverse-plane balance between H → bb and H → γγ, exploits the excellent H → γγ reconstruction to improve the H → bb resolution through event-level likelihood maximization. The final statistical results are obtained by applying the full analysis workflow to the bbγγ channel, incorporating b-jet corrections from the Kinematic Fit technique.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
