Hidden sector or dark sector states appear in many extensions to the Standard
Model (SM), to provide particle mediators for dark matter in the universe or to
explain astrophysical observations such as the positron excess in the cosmic
microwave background radiation flux. A hidden or dark sector can be introduced
with an additional U(1)d dark gauge symmetry. The discovery of the Higgs boson
in 2012 during Run 1 by the Large Hadron Collider (ATLAS and CMS) opens a new
and rich experimental program for Beyond Standard Model physics (BSM) based
on the Higgs Portal. This exotic discovery route uses couplings to the dark sector at
the Higgs level, which were not experimentally accessible before. This thesis
presents the searches of possible exotic decays: H → ZdZ(d) → 4` where Zd
is a
dark vector boson. It had been initiated in the Run 1 period of the LHC using the
ATLAS detector at CERN. The results showed (tantalizingly) two signal events
where none were expected, so that in the strict criteria of High Energy Physics, the
result was not yet statistically significant. The Run 1 analysis for a 8 TeV collision
energy is further developed in Run 2 with a 13 TeV collision energy, to expand the
search area, take advantage of higher statistics, a higher Higgs production cross
section, and substantially better performance of the ATLAS detector. In this work,
the search is further broadened and includes allowing the mass of the originating
boson (the dark Higgs S) to vary from the SM value. This allows the search for the
dark vector boson to also explore higher or lighter masses than the SM Higgs
boson. This extended search is efficient and could include a more general class of
models, with the mass constraint of the SM Higgs portal lifted. This thesis reviews
the analysis results from Run 1 and Run 2, and presents its iteration in the full Run
2 search by focusing on its new channel where the additional scalar S
(with mS 6= mH) decays to 4` via two dark vector boson states Zd
. The case where
the Higgs decays to 4` via two Zd
(H → ZdZd → 4`) and also called high mass
channel, has been just unblinded. Nineteen data events are observed where 14 were
predicted. In overall, the data are consistent with the Monte Carlo prediction. No
evidence of deviation from the Standard Model expectations are observed. / College of Engineering, Science and Technology / Ph. D. (Physics)
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:unisa/oai:uir.unisa.ac.za:10500/26696 |
| Date | 06 1900 |
| Creators | Boye, Diallo |
| Contributors | Leeuw, Lerothodi L., Connell, S. H. (Simon H.), Assamagan, Ketevi, Truong, Loan |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 1 online resource (xxvii, 158 leaves) ; color illustrations, color graphs, application/pdf |
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