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Martinez Outschoorn, Verena Ingrid Pitts, Kevin Peng, Jen-Chieh Shelton, Jessie Coelho Lopes de Sa, Rafael Cai, Huacheng 2020-08-26T21:54:08Z 2020-08-26T21:54:08Z 2020-03-19 2020-05 Searches for exotic Higgs boson decays are well motivated by various theoretical models, as well as the constraints from Higgs boson measurement results. The focus is on exotic Higgs boson decays to a pair of beyond-the-Standard-Model spin-0 particles $a$. This dissertation conducts two analyses for exotic Higgs bosons decaying into leptons, which are $H\rightarrow2a\rightarrow4\mu$ and $H\rightarrow2a\rightarrow4\tau$, and target the mass ranges of $1<m_a<15\text{ GeV}$ and $15<m_a<60\text{ GeV}$. Various data-driven techniques are developed and used to estimate the background that cannot be well modeled with the standard simulation in ATLAS. The searches use proton-proton collision data collected with the ATLAS detector at the LHC with an integrated luminosity of $139\text{ fb}^{-1}$ and a center of mass energy $\sqrt{s}=13\text{ TeV}$. No significant excess has been observed. Therefore, this dissertation summarizes the $95\%$ confidence-level upper limits of the branching ratio of exotic Higgs decays to a pair of beyond-the-Standard-Model light scalars in the lepton final states Submission original under an indefinite embargo labeled 'Open Access'. The submission was exported from vireo on 2020-08-25 without embargo terms The student, Huacheng Cai, accepted the attached license on 2020-03-16 at 11:31. The student, Huacheng Cai, submitted this Dissertation for approval on 2020-03-16 at 11:33. This Dissertation was approved for publication on 2020-03-19 at 11:52. DSpace SAF Submission Ingestion Package generated from Vireo submission #14901 on 2020-08-25 at 17:05:51 Made available in DSpace on 2020-08-26T21:54:08Z (GMT). No. of bitstreams: 3 CAI-DISSERTATION-2020.pdf: 15302488 bytes, checksum: 4caf5f09d86e72a5da39fe689d7c98f6 (MD5) LICENSE.txt: 4209 bytes, checksum: 345f982bb6eb980dfe5214e247e4a3e7 (MD5) PROQUEST_LICENSE.txt: 4555 bytes, checksum: e487f6a1b46d2eb89c816104726919ba (MD5) Previous issue date: 2020-03-19 application/pdf http://hdl.handle.net/2142/107860 en Copyright 2020 Huacheng Cai high energy physics experiments Higgs physics beyond Standard Model physics LHC physics ATLAS detector Search for exotic Higgs boson decays to four leptons with the ATLAS detector text Thesis Physics Physics University of Illinois at Urbana-Champaign Dissertation Ph.D.