Exploring extended Higgs sectors and the CP Nature of the Higgs Yukawa coupling using tau leptons at the CMS experiment
File(s)
Author(s)
Savva, Klitos
Type
Thesis
Abstract
Since the discovery of the Higgs boson at the LHC, attention has turned to testing its properties and searching for extended Higgs sectors that could reveal new physics. This thesis addresses three complementary topics within the CMS experiment: the study of a compact data format to meet the ever-growing computing needs, a direct search for additional Higgs bosons in the four-tau final state, and a precision measurement of the CP structure of the Higgs–tau Yukawa coupling. First, the development of the compact RAW' data format is described, demonstrating recovered performance in displaced track reconstruction and positioning RAW' as a promising option. Second, a search for additional Higgs bosons in the four-tau final state is performed using the full Run 2 dataset of $137\unit{fb}^{-1}$ at $\sqrt{s}=13\TeV$. No excess is observed, and exclusion limits on $Z^* \to \phi A \to 4\tau$ production reach from $\mathcal{O}(190\unit{fb})$ at low masses to $\mathcal{O}(0.4\unit{fb})$ at high masses. Regions of the Type-X 2HDM relevant to the muon anomalous magnetic moment are also excluded. Finally, a measurement of the CP structure of the Higgs–tau Yukawa coupling is performed in the $\tauh\tauh$ channel with Run 3 data, excluding the pure CP-odd hypothesis at $1.74\sigma$. When combined with the Run 2 result, the exclusion improves to $3.55\sigma$ with $\alpha^{\PH\tau\tau}$ constrained to $\pm 15^\circ$, representing the most precise determination to date. This result remains compatible with the SM prediction of a CP-even coupling.
Date Issued
2025-09-30
Date Awarded
01/01/2026
License URL
Advisor
Colling, David
Publisher Department
Department of Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)