Search for physics beyond the standard model at particle colliders using machine learning techniques

Both ATLAS and CMS collaborations have reported the discovery of a res- onance with a mass around 125 GeV, immediately associated with the long sought after Higgs boson. With this discovery, the standard model (SM) of particle physics is considered to be complete. However, non-vanishing neu- trino masses provide the first confirmed hint towards physics beyond the SM. Extending the SM by sterile neutrinos can naturally explain the smallness of neutrino masses as observed by neutrino oscillation. Moreover, the problem of naturalness and the meta stability of the SM vacuum are hints for extending the SM scalar potential. A well motivated framework to naturally extend the SM scalar sector is to add SU(2)L complex scalar singlet, doublet or triplet to the SM Lagrangian. These hypothetical particles, the additional neutrinos or scalars, can give rise to a testable phenomenology when they have masses around the electroweak scale. They are actively searched for at the current colliders such as the Large Hadron Collider (LHC). The proposed future col- liders, which are currently in the design phase, will be more powerful than the operated colliders to date.
In this dissertation, we investigate various aspects of sterile neutrino and heavy scalar phenomenology as well as the prospects of the current and fu- ture colliders searches for them. In particular, we investigate the prospects of searching for sterile neutrinos in low scale seesaw scenarios via the lepton fla- vor violating (but lepton number conserving) dilepton dijet signature at the high luminosity phase of the current LHC and the proposed Large Hadron electron Collider (LHeC). For part of the parameter space where the sterile neutrino is long lived, where the sterile neutrino mass is less than the W bo- son mass, we investigate the LHeC sensitivity to the displaced vertex search. For sterile neutrino mass above O(TeV) we investigate the LHeC sensitivity to the charged lepton flavor violation processes that can be generated at one loop level. We show that for all sterile neutrino mass range, from 5 GeV up to O(105) GeV, the LHeC could already probe the LFV signatures beyond the current experimental bounds.
In the second part of this dissertation, various aspects for heavy scalars ex- tending the SM Lagrangian and various searches for heavy scalars at colliders are investigated. We start with studying the prospects of the LHeC search for heavy neutral scalars within the minimal extension of the SM Lagrangian with one additional complex scalar field. The mixing between the singlet and the SM doublet scalar fields gives rise to a SM like Higgs and a heavy scalar. More- over, it induces phenomenological signatures for the heavy Higgs, analogously to the SM Higgs boson, that can be tested at the LHeC. Using multivariate anal- ysis and machine learning techniques we show that the LHeC could probe the heavy signatures for masses between 200 and 800 GeV beyond the current LHC and its future high luminosity phase sensitivity.
Also, the prospect signature of the spontaneous CP violation are investigated via the angular distribution of tau lepton pair produced from heavy scalar decays. For this purpose, we focus on the Two Higgs Doublet Model, as an ex- ample model, which can offer a source for CP violation in its scalar potential. Considering a benchmark point that is compatible with the current constraints but within reach of the high luminosity LHC, we study the prospects of de- termining the CP property of a heavy neutral Higgs state. We show that CP conservation in the scalar sector can be excluded at the 90% CL for a bench- mark point with maximal CP violation if the background can be controlled with a relative accuracy of 0.5%, which could be the accuracy target for future LHC measurements.
Finally, we study the phenomenology of the minimal extension of the SM La- grangian with a scalar triplet field. The precedence of this scenario over any other consideration that after the symmetry breaking, when the components of the triplet field have masses around the electroweak scale, the model fea- tures a rich phenomenology. We discuss the current allowed parameter space taking into account all relevant constraints, including charged lepton flavour violation as well as collider search. In part of the parameter space the triplet components can be long lived, with displaced distance, in the range of cm, potentially leading to a characteristic displaced vertex signature where the doubly charged component decays into same sign charged leptons. The fact that the LHC is looking for doubly charged scalars via track only analysis or charged stable particles, makes this part of the parameter space still untested by the current LHC measurements. By performing a detailed analysis at the reconstructed level we show that already at the current run of the LHC a dis- covery would be possible for the considered parameter point, via dedicated searches for displaced vertex signatures.