Simplified Models and Effective Theories in Searches for New Physics

In this thesis two different types of effective field theories (EFTs) have been considered to give an interpretation to effects of physics beyond the Standard Model. The first type of EFT is a so-called Simplified Models, in which the Standard Model is extended by new scalar degrees of freedom (CP-odd) and (CP-eve), which give rise to interactions of the type and with subsequent decay of . We study the phenomenology of these processes at the LHC and find that the production of is suppressed compared to that of . Using the Weyl-van-der-Waerden spinor formalism we analyse the helicity amplitudes in order to explain this phenomenon. In the second part of this thesis, we focus on the dimension-six Standard Model Effective Field Theory (SMEFT) to calculate the next-to-leading order (NLO) electroweak corrections to the forward-backward asymmetry in the process . We find that the NLO EFT corrections can not be neglected compared to the LO EFT contributions. We show that relevant numerical results have been obtained, which can be used to constrain the Wilson Coefficients involved in this process. We calculate the amplitude of the process and renormalise it together with the process. We have also analysed the NLO EFT corrections to the cross-section of the process, which can be used to set limits on the Wilson coefficients with future collider experiments.