자료유형  

 학위논문

Control Number  

0017162834

International Standard Book Number  

9798382739670

Dewey Decimal Classification Number  

530

Main Entry-Personal Name  

Tsai, Meng-Ju.

Publication, Distribution, etc. (Imprint  

[S.l.] : University of Michigan., 2024

Publication, Distribution, etc. (Imprint  

Ann Arbor : ProQuest Dissertations & Theses, 2024

Physical Description  

267 p.

General Note  

Source: Dissertations Abstracts International, Volume: 85-12, Section: B.

General Note  

Advisor: Qian, Jianming.

Dissertation Note  

Thesis (Ph.D.)--University of Michigan, 2024.

Summary, Etc.  

요약This dissertation presents the measurements and searches of four-top-quark (tt¯t t¯) production with same-sign dilepton and multilepton final states in proton-proton collision data collected using the ATLAS detector at the LHC. Four-top-quark production, a rare process in the Standard Model (SM) with a cross section around 12 fb at a center-of-mass energy of 13 TeV, is one of the heaviest final states produced at the LHC, and it is naturally sensitive to beyond SM (BSM) physics. An early analysis by the ATLAS experiment (referred to as the tt¯t t¯ evidence analysis) resulted in an evidence for SM tt¯t t¯production with a significance of 4.3 standard deviations with respect to the background-only hypothesis, but the measured tt¯t t¯ cross section is twice the SM prediction. This dissertation focuses on three major topics on the tt¯t t¯production: the observation of the SM tt¯t t¯production, a search of BSM tt¯t t¯production (tt¯H/A → tt¯t t¯) predicted in the type-II Two-Higgs-Doublet Model (2HDM), and a reinterpretation analysis of BSM tt¯t t¯ search in a 2HDM+a combination analysis.The first is the observation of four-top-quark production with same-sign dilepton and multilepton final states in an integrated luminosity of 140 fb−1 of proton-proton collision data at a center-of-mass energy of 13 TeV collected using the ATLAS detector. The analysis is based on the tt¯t t¯ evidence analysis with additional optimizations on object selection, background estimation, and a novel multivariate technique. Looser lepton identification requirements and lower jet pT criteria are employed to maximize the tt¯t t¯ signal acceptance. A new tt¯W data-driven background estimation is utilized to improve the modeling of tt¯W jet multiplicity distribution. A novel graph neural network is used to separate the signal from the background. A simultaneous fit to both signal and control regions is performed to extract the signal strength and constrain various background contributions. The tt¯t t¯ cross section is measured to be 22.5 +6.6−5.5 fb, consistent with the SM prediction of 12.0 ± 2.4 fb within 1.8 standard deviations. The corresponding observed (expected) significance of tt¯t t¯ production is found to be 6.1 (4.3) standard deviations with respect to the background-only hypothesis. Additional interpretations are explored, including limits on three-top-quark production, constraints on the Higgs-top Yukawa coupling, and effective field theory operator coefficients affecting tt¯t t¯ production.The second is the search for a new heavy scalar or pseudo-scalar Higgs boson (H/A) predicted by the type-II 2HDM. The heavy Higgs boson is produced in association with a pair of top quarks (tt¯H/A ), with the Higgs boson decaying into a pair of top quarks (H/A → tt¯), leading to tt¯t t¯ production (tt ¯H/A → tt¯t t¯). This analysis (referred to as the BSM tt¯t t¯analysis) targets same-sign dilepton and multilepton final states with data corresponding to an integrated luminosity of 139 fb−1 . The analysis is based on the tt¯t t¯ evidence analysis with improved jet and b-jet algorithms. Two classifiers using Boosted Decision Trees are utilized to discriminate the signal and backgrounds. No significant excess of events is observed over the SM prediction. The observed (expected) upper limit at 95% confidence level on the tt¯H/A cross section times the branching ratio of H/A → tt¯ ranges between 14 (10) fb and 6 (5) fb for a heavy Higgs boson mass of 400 GeV and 1000 GeV. Constraints on tan β are derived under different scenarios. Assuming only one particle, either H or A, contributes to tt¯t t¯ production, tan β values below 1.2 or 0.5 are excluded for a heavy Higgs mass of 400 GeV or 1000 GeV. Assuming both H and A particles contribute to tt¯t t¯production, tan β values below 1.6 or 0.6 are excluded for a heavy Higgs mass of 400 GeV or 1000 GeV.The third is the combination of BSM searches to constrain the parameters of type-II 2HDM with an additional pseudo-scalar (a) mediating the interaction between ordinary matter and dark matter. The tt¯t t¯ process can be produced either from an s-channel diagram through a heavy Higgs boson (H/A/a) decaying to a pair of top quarks or a t-channel diagram coupling with a heavy Higgs boson. This analysis follows the same strategy as the BSM tt¯t t¯ analysis to reinterpret with tt¯t t¯ production predicted in 2HDM+a, but uses different final discriminating distributions in the signal region. Constraints on the parameters of 2HDM+a are derived in several benchmark scenarios.

Subject Added Entry-Topical Term  

Physics.

Subject Added Entry-Topical Term  

Particle physics.

Subject Added Entry-Topical Term  

Atomic physics.

Subject Added Entry-Topical Term  

Computational physics.

Index Term-Uncontrolled  

ATLAS experiment

Index Term-Uncontrolled  

Top quark physics

Index Term-Uncontrolled  

High energy experiment

Index Term-Uncontrolled  

Four-top-quark

Index Term-Uncontrolled  

Same-sign dilepton

Added Entry-Corporate Name  

University of Michigan Physics

Host Item Entry  

Dissertations Abstracts International. 85-12B.

Electronic Location and Access  

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Control Number  

joongbu:658620