Naturalness in beyond the standard model physics

Being consistent with every experimental measurement made to date, the current paradigm of particle physics, the Standard Model, remains a successful parametrization of nature. Together, the Standard Model plus the theory of General Relativity seem to provide a consistent picture of physics at all scales, yet there is plenty of room to believe the story is incomplete. Puzzles that remain unanswered within the context of the Standard Model include (i) an explanation of the origin of Dark Matter, which accounts for no less than ∼ 25% of the Universe’s energy budget, (ii) a meaningful answer to the question of electroweak naturalness, (iii) a rationale for the absence of anti-matter in our observable patch of the Universe, (iv) a dynamical picture of the vast hierarchies we observe in fermion masses, and (v) a resolution to the strong CP problem. With the exception of the Dark Matter mystery, all other objections to the Standard Model listed here take the guise of a 'hierarchy' problem: why is some quantity (either a scale or coupling) so small? This work addresses two of the objections to the Standard Model: the necessity of an explanation to the origin of Dark Matter, and the question of naturalness as a guiding principle in nature, understood as the necessity for a dynamical mechanism behind unexplained hierarchies. Chapters 1 and 2 introduce the topics of naturalness and Dark Matter respectively. The former makes an emphasis on the electroweak hierarchy problem, and a particular class of theories that provide a solution to this puzzle: models based on the Twin Higgs mechanism. Chapters 3 and 4 are based on work published in [1] and [2], where novel theories of Dark Matter, and their phenomenology, are explored in the context of Twin Higgs models. Chapter 5 explores structural aspects of a particular mechanism – the so-called 'clockwork' – for generating hierarchies in parameters in a way that can be considered natural, and it is based on [3]. Finally, chapter 6 summarizes our conclusions and future outlook. Other work published during my time as a graduate student include [4–7], but those publications are not the focus of this thesis.