Sharif Digital Repository

Effective field theory is a practical method for model building and explaining experimental/observational data. These theories are constructed based on knowing low energy degrees of freedom and symmetries (accidental or fundamental). By construction, these theories are applicable up to a cut-off scale and their predictions beyond that are not reliable. For applications to higher energies a UV completion is needed through which modifications are applied or new heavy degrees of freedom are introduced. In order to complete a theory in a Wilsonian approach, scattering amplitudes must satisfy some constraints. These constraints are derived from basic assumptions like Lorentz invariance,... 

In this thesis, we first explain how we can construct three-point amplitudes for both massive and massless particles by solely knowing the Lorentz invariance and spin or helicity of particles. Then, using BCFW recursion relation, we determine the tree-level amplitudes. To calculate the loop amplitude, we apply the unitarity of Smatrix.The unitarity relates the tree-level amplitudes to loop level. Finally, I discuss some exciting topics in this field, including color-kinematics duality, some relevant applications of contemporary S-matrix theory in effective field theories