New Approaches to Beyond Standard Model Searches with Colliders and Cosmology

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The Large Hadron Collider (LHC) era presents us with a puzzle. Extensions to the Standard Model (SM) are as urgently needed as ever, but none of the canonically expected signals have shown up so far. Resolving this conundrum requires new perspectives on what types of signals are theoretically motivated and new approaches to experimental searches. This challenge and opportunity provides the unifying theme of my research in phenomenology. Long-lived particles (LLPs) produced at colliders generically arise in many Beyond SM (BSM) scenarios and could reveal the secrets of naturalness, dark matter, and baryogenesis. In the past, the focus on signals inspired by minimal supersymmetry meant that LLPs received much less attention. This is now changing, in significant part due to my work. I am spearheading research into LLP signals at the LHC and am collaborating with experimentalists and theorists to help determine the roadmap of BSM searches for the coming decade. Born from these investigations is our MATHUSLA detector proposal, which can increase LHC sensitivity to LLP signals by three orders of magnitude. Progress has been swift, and I am now helping to lead the efforts of over 100 theorists and experimentalists to make this idea a reality. Given MATHUSLA's potential for discovery the importance of this endeavor cannot be overstated, making it a major focus for me in the coming years. It also represents a unique opportunity for Canada to take on a leadership position in fundamental particle physics. The discovery of the Higgs boson has opened up many exciting avenues of investigation for both SM and BSM physics. Hidden Sectors can lead to a plethora of as-yet undetected collider signals, modify cosmology and even impact astrophysics. They are theoretically motivated for a variety of top-down and bottom-up reasons, but recently their potential to address the hierarchy problem has come into focus. Exotic Higgs decays are a unique portal to these theories at the LHC. Theories of Neutral Naturalness lead to spectacular LLP signals and are a major motivation for their study, but many aspects of their nonperturbative dynamics, theoretical UV completions, and cosmological consequences are not understood. Study of the Higgs also allows us to begin probing the Electroweak Phase Transition, which is of fundamental importance for our understanding of cosmology, and may play a role in explaining the universe's matter-antimatter asymmetry via the mechanism of Electroweak Baryogenesis. This requires the introduction of new theoretical tools to deal with challenging aspects of Finite-Temperature Quantum Field Theory. Only by making calculations robust and model-independent can we conclusively discover or exclude these and other important theoretical ideas. Finally, many BSM scenarios can only be probed at future colliders, and articulating this physics case will determine the future of our field in the coming decades.