Photon-Photon processes at the International Linear Collider and BSM signatures with small mass differences

The International Linear Collider is a proposed $e^{+}e^{-}$ collider with tunable centre-of-mass energies and polarised beams. By performing high-precision measurements of Standard Model observables and searches for new particles the ILC can serve as a complementary machine to the LHC.In supersymmetric extensions of the Standard Model, higgsino-like charginos and neutralinos are preferred to have masses of the order of the electroweak scale by naturalness arguments. Such light $\tilde{\chi}_{1}^{\pm}$, $\tilde{\chi}^{0}_{1}$ and $\tilde{\chi}^{0}_{2}$ states can be almost mass degenerate. Due to their mass degeneracy it is very difficult to observe the decay of such higgsinos at hadron colliders. ILC being an $e^+e^-$ collider has the prospect of providing a very clean physics environment to observe or exclude such scenarios. However, in addition to the desired $e^{+}e^{-}\rightarrow\tilde{\chi}^{+}\tilde{\chi}^{-}$ processes, parasitic collisions of real and virtual photons radiated off the $e^+e^-$ beams occurat the rates depending on the centre-of-mass energy (250 GeV - 1 TeV) and other beam parameters. In this thesis the effects of such $\gamma\gamma\rightarrow$ low $p_{T}$ hadron overlay on the low $\Delta M$ higgsino analysis is studied.The study considers two benchmark points that exhibits mass differences of $\mathcal{O}$ [1 GeV] in the higgsino sector. In the given higgsino scenarios, the visible decay products have low transverse momenta due to the small mass differences between the higgsinos. The $\gamma\gamma\rightarrow$ low $p_{T}$ hadron overlay has a very similartopology to the signal event which makes the removal of the overlay very challenging.The standard methods to remove $\gamma\gamma$ background, e.g methods based on jet clustering remain inadequate. The study presents a newly developed track grouping algorithm which is based on the concept of displaced signal and $\gamma\gamma\rightarrow$ low $p_{T}$ hadron overlay vertices. By applying the track grouping algorithm to separate $\gamma\gamma\rightarrow$ low $p_{T}$ hadron tracks from the higgsino decay tracks, ananalysis has been performed using the full detector simulation for the International Large Detector (ILD). It is shown that the group tracking algorithm can very efficiently separate $\gamma\gamma\rightarrow$ low $p_{T}$ hadron tracks from the higgsinodecay tracks. The results from the analysis show that even with the presence of $\gamma\gamma\rightarrow$ low $p_{T}$ hadron overlay, the key observables of the higgsinos can be reconstructed with an uncertainty of a few percent. A comparison with the previous study which was performedwithout the inclusion of $\gamma\gamma\rightarrow$ low $p_{T}$ hadron events is made to enhance the understanding about the effects of the overlay on the higgsino analysis