Hunting for New Physics in New Places.

Abstract

In this thesis, we consider two questions concerning the nature of physics beyond the Standard Model (BSM). First, to what extent are minimal or favored BSM models under pressure due to recent experimental results? Second, how might we observe new physics experimentally?

Motivated by naturalness, we assume that new particles responsible for stabilizing the weak-scale against radiative corrections will soon be discovered at the LHC. Specifically, we consider the case in which these will be the fermionic top partners of a Little Higgs (LH) model. We argue that a slew of exotic top partners beyond the minimal set required by naturalness should exist. In addition, we enumerate the pseudo Nambu-Goldstone Bosons (pNGBs) that can be present in a LH model beyond a single Higgs doublet, and explain how these pNGBs can be produced in exotic top partner decays. We perform a phenomenological study of these decays, and find that they may present the most promising avenue for observing extended Higgs sector states. Though our study is motivated by LH models, our results are applicable to a variety of models with fermionic top partners that decay to (non-Higgs) scalars.

We then turn our attention to dark matter (DM). We examine the extent to which a strictly weakly-interacting massive particle (WIMP) --- i.e. one whose phenomenology is controlled by the bosons of the electroweak theory --- remains a viable DM candidate in light of recent direct detection, indirect detection and collider searches. Assuming a thermal cosmological history, such a WIMP is already tightly constrained, and future experiments will probe much of the remaining parameter space. However, these constraints could be avoided if the dark sector particles exhibit certain mass relations. We propose that such dark sector mass relations could arise as a result of renormalization group focusing, and point out that achieving the desired focusing may require the existence of additional particles or interactions that could yield novel phenomenology in lieu of WIMP detection.