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

Abstract

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 higgsino 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 higgsino processes, parasitic collisions of real and virtual photons radiated off the e+e− beams occur at the rates depending on the centre-of-mass energy (250 GeV - 1 TeV) and other beam parameters. In this thesis the effects of such γγ → low pT hadron overlay on the low ∆M higgsino analysis is studied. The study considers two benchmark points which exhibits mass differences of 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 γγ→ low pT hadron overlay has a very similar topology to the signal event which makes the removal of the overlay very challenging. The standard methods to remove γγ background, e.g methods based on jet clustering remain in-adequate. The study presents a newly developed track grouping algorithm that is based on the concept of displaced signal and γγ→ low pT hadron overlay vertices. By applying the track grouping algorithm to separateγγ→low pT hadron tracks from the higgsino decay tracks, an analysis 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 γγ→ low pT hadron tracks from the higgsino decay tracks. The results from the analysis show that even with the presence of γγ → low pT 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 performed without the inclusion of γγ→ low pT hadron events is made to enhance the understanding of the effects of the overlay on the higgsino analysis.