Phenomenological aspects of the E6SSM.
University of Southampton, Faculty of Physical and Applied Sciences: Theoretical Partical Physics Group,
The work in this thesis explores various phenomenological aspects of the E6SSM with a particular focus on the inert neutralino sector of the model and on the dark matter implications. The E6SSM is a string theory inspired supersymmetric extension to the Standard Model with an E6 grand unification group. The model provides a solution to the hierarchy problem of the Standard Model, provides an explanation for neutrino mass, and has automatic gauge anomaly cancellation. The inert neutralino sector of the E6SSM and the dark matter that naturally arises from this sector is studied for the first time. Limits on the parameter space from experimental and cosmological observations relating to the inert neutralino dark matter are explored and the consequences for Higgs boson phenomenology are investigated. In plausible scenarios it is found that the couplings of the lightest inert neutralinos to the SM-like Higgs boson are always rather large. This has major implications for Higgs boson collider phenomenology and leads to large spin-independent LSP-nucleon cross-sections. Because of the latter, scenarios in which E6SSM inert neutralinos account for all of the observed dark matter are now severely challenged by recent dark matter direct detection experiment analyses. In plausible scenarios consistent with observations from both cosmology and LEP the lightest inert neutralino is required to have a mass around half of the Z boson mass if it contributes to cold dark matter and this means that tan(β) cannot be too large. A new variant of the E6SSM called the E6ZS 2 SSM is also presented in which the dark matter scenario is very different to the inert neutralino cold dark matter scenario and in which the presence of supersymmetric massless states in the early universe modifies the expansion rate of the universe prior to Big Bang Nucleosynthesis. The new dark matter scenario is consistent with current observations and the modified expansion rate provides a better explanation of various data than the SM prediction. The prospects for a warm dark matter scenario in the E6SSM are also briefly discussed
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