Strongly Coupled Models at the LHC

In this thesis strongly coupled models where the Higgs boson is composite are discussed. These models provide an explanation for the origin of electroweak symmetry breaking in- cluding a solution for the hierarchy problem. Strongly coupled models provide an alternative to the weakly coupled supersymmetric extensions of the Standard Model and lead to dif- ferent and interesting phenomenology at the Large Hadron Collider (LHC). This thesis discusses two particular strongly coupled models, a composite Higgs model with partial compositeness and the Littlest Higgs model with T-parity | a composite model with col- lective symmetry breaking. The phenomenology relevant for the LHC is covered and the applicability of eective operators for these types of strongly coupled models is explored. First, a composite Higgs model with partial compositeness is discussed. In this model right-handed light quarks could be signicantly composite, yet compatible with experi- mental searches at the LHC and precision tests on Standard Model couplings. In these scenarios, which are motivated by avour physics, large cross sections for the production of new resonances coupling to light quarks are expected. Experimental signatures of right- handed compositeness at the LHC are studied, and constraints on the parameter space of these models are derived using recent results by ATLAS and CMS. Furthermore, dedicated searches for multi-jet signals at the LHC are proposed which could signicantly improve the sensitivity to signatures of right-handed compositeness. The Littlest Higgs model with T-parity, providing an attractive solution to the ne-tuning problem, is discussed next. This solution is only natural if its intrinsic symmetry breaking scale f is relatively close to the electroweak scale. The constraints from the latest results of the 8 TeV run at the LHC are examined. The model's parameter space is being excluded based on a combination of electroweak precision observables, Higgs precision physics and direct searches at the LHC. These tests provide stringent limits on f and the parameter space is slowly driven into the TeV range. Furthermore, a strategy on how to optimise present supersymmetry searches for the Littlest Higgs model with T-parity is presented, with the goal to improve the constraints and yield more stringent limits on f . Finally, the robustness of translating eective operator constraints to beyond the Standard Model (BSM) theories is treated and turns out to crucially depend on the mass and coupling of BSM particles. This is especially relevant for hadron colliders where the partonic centre of mass energy is around the typical energy scales of natural BSM theories. The caveats in applying limits from eective operators are discussed using Z 0 and G 0 models, illustrating the eects for a large class of models. This analysis shows that the applicability of eective operators mainly depends on the ratio of the transfer energy in the events and the mass scale of the full theory. Moreover, based on these results a method is developed to recast existing experimental limits on eective operators to the full theory parameter space. It is concluded that strongly coupled models of electroweak symmetry breaking are still natural and compatible with LHC results. Moreover, these types of models provide new and interesting nal state topologies for experimental searches at the LHC. For the high energy runs of the LHC these new searches will prove useful in determining the faith of composite models and maybe thereby the origin of electroweak symmetry breaking.