Thesis defense of Daniel Wendler

Missing energy signatures provide a broad probe of physics beyond the Standard Model, arising both from modified interactions of Standard Model neutrinos and from new invisible particles. In this thesis, we study missing energy phenomenology, focusing on the Standard Model Effective Field Theory (SMEFT) and its extension by a Standard Model-singlet fermion. In the SMEFT, we derive general expressions for inclusive Drell--Yan processes and use current LHC searches with missing energy and (un)tagged jets to constrain dimension-six interactions. Depending on the operator and flavor structure, current data probe effective new-physics scales of approximately $\SIrange{1}{5}{\TeV}$, while future hadron colliders can improve the reach by up to a factor of eight. We then consider a singlet fermion $N$, which can be used to construct operators that can violate visible or total baryon number and lead to missing energy, displaced-vertex, or prompt multijet signatures depending on the mass and lifetime of $N$. Current missing energy data probe effective scales up to about $\SI{11}{\TeV}$, with displaced-vertex searches providing sensitivity for $M_N \sim \SI{10}{\GeV}$ and multijet searches constraining scales up to a few $\si{\TeV}$ for $M_N \gtrsim \SI{100}{\GeV}$. We further connect these collider constraints to low-energy baryon and top-quark decays, explicit ultraviolet completions, and B-Mesogenesis. Overall, the results demonstrate how missing energy observables provide a common framework for probing different realizations of invisible new physics across energy scales.