Direct and Indirect Search for New Physics at Colliders

The Standard Model (SM) of particle physics has been remarkably successful to describe strong, weak and electromagnetic forces of nature, as it has agreed with most of the experiments till date. However, there are still some unanswered questions both theoretical and phenomenological in the SM. The present and future colliders will help the physicists learn more about the nature of matter and all forces in the universe. In this thesis work we have mainly focused on the physics case of one of the future linear colliders which has been proposed and will hopefully be a succession of the currently running Large Hadron Collider (LHC) in CERN Geneva. As an introduction to the thesis work in Chapter 1 we have discussed in detail about the proposed future collider, namely the International Linear Collider (ILC). This collider alongside being a high luminosity machine, will have the advantage of beam polarization as the colliding beams will be linear in this case. In addition, as the initial particles are e+ and e􀀀, the collision will take place in a clean environment, with almost negligible QCD background. As physics at the ILC has major dependence on beam polarization, we discuss here the basic physics of beam polarization, helicity amplitudes etc. We also discuss about the basic structure of the Standard Model, Higgs mechanism and explore its unanswered questions. One of the theories proposed to take care of these deficiencies, namely super symmetry has been discussed in this chapter. These theories apart from explaining the shortcomings of the SM, also predict many new particles and are thus phenomenologically rich. There are two ways to look for the physics beyond SM. One is direct search for the new states which are not present in SM, which is the goal of the currently running collider LHC. This is a very efficient way if the new particle state is within the reach of the energy of the collider. The other is an indirect way, where deviations from SM is studied by a thorough scrutinization of the SM processes even without direct detection of the new physical states. The goal of thesis is the search for new physics scenarios in both direct and indirect ways. The full potential of the ILC through initial beam polarization, has been explored in this thesis. The work can be divided into two basic themes pertaining to the effective _field theory (EFT) approach of the search for beyond Standard Model (BSM) physics in the top and gauge sector and the study of Higgs phenomenology in the colliders. Below, these works will be described in some detail and the future research plans related to them. In Chapter 2, we have discussed model independent EFT analysis of the search for new physics in the process e+e􀀀! tt at the ILC. We consider four-Fermi type interaction and look at the effect of different type of new physics (scalar, tensor, vector or axial vector) in addition to the V-A type interaction of SM. We consider initial beams to be polarized (longitudinal or transverse). The availability of beam polarization at the ILC…