Investigating Custodial Symmetry Violation in the Georgi-Machacek Model

The Georgi-Machacek (GM) model is the simplest benchmark model for higher isospin multiplet extensions of the Standard Model (SM) which allow for large contributions to electro-weak symmetry breaking (EWSB) from these 'exotic' multiplets. It is used to motivate and interpret Large Hadron Collider (LHC) searches for doubly charged scalars decaying to vector boson pairs. It preserves custodial symmetry (CS) at tree level; however, this CS has long been known to be violated at the one-loop level by hypercharge interactions. In this thesis, we study the effects of CS violation (CV). We assume that the CS GM model arises at a high scale as a result of an unspecified ultraviolet (UV) completion, and quantify the CV induced at the weak scale. We show that experimental data constrains the UV scale to lie below tens to hundreds of TeV over almost all of the parameter space. Subject to this constraint, we quantify the size of other CV effects at the weak scale and we find that these effects are small enough that they are unlikely to be probed by the LHC, but may be detectable at a future e+e- collider. We note that the upper bound on the UV scale is large enough that virtual effects from the UV completion will likely be undetectable at the LHC. This means that the GM model is a valid effective theory for LHC physics. We also propose a new "low-m5" benchmark for the tree level GM model, defined for m5 2 (50; 550) GeV, and characterize its properties. This benchmark plane is designed to facilitate the extension of current collider searches for doubly charged scalars to an unexplored region of parameter space which was not previously covered by a benchmark plane. The doubly charged scalars are part of a degenerate fermiophobic custodial fiveplet with states H_5^{++} , H_5^+, and H_5^0 and the input parameter m5 is their common mass. We apply all existing experimental constraints and summarize the phenomenology of the surviving parameter space. We show that there is a largely unprobed region of parameter space from 120 GeV < m5 < 200 GeV where light doubly-charged scalars could exist.