We explore a mechanism for producing the baryon asymmetry and dark matter in models with multiple hidden sectors that are Standard Model-like but with varying Higgs mass parameters. If the field responsible for reheating the Standard Model and the exotic sectors carries an asymmetry, it can be converted into a baryon asymmetry using the standard sphaleron process. A hidden sector with positive Higgs mass squared can accommodate dark matter with its baryon asymmetry, and the larger abundance of dark matter relative to baryons is due to dark sphalerons being active all the way down the hidden sector QCD scale. This scenario predicts that dark matter is clustered in large dark nuclei and gives lower bound on the effective relativistic degrees of freedom, ∆N eff 0.05, which may be observable in the next-generation Cosmic Microwave Background experiment CMB-S4.

We consider experimental limits on colour triplet fermions that decay dominantly to three jets via a scalar mediator that can be on- or off-shell. These fermions arise in top-partner models that can solve the hierarchy problem, and limits on this scenario are weaker than those on traditional top-partner models because of the messy all-hadronic final state with significant backgrounds. We do find, however, that while there are no dedicated searches for this scenario, especially in case of an on-shell mediator, the suite of LHC all- hadronic searches still constrains a significant portion of the parameter space. In particular, we find that searches for pair production of di-jet and tri-jet resonances are complementary, covering different regions of parameter space. We also find that if the final state is rich in b- jets, current limits do not change significantly relative to the scenario with all light jets, and we describe how modifications of current search strategies can improve limits in that case.

This thesis explores some extensions of the Standard Model in search of new physics.The research presented here concerns aspects of cosmology and collider physics. The research includes a combination of model building, constraints imposed by the current experimental data, and prospects of future experimental bounds. There is an aggregate of three main research projects composing this thesis.Firstly, the possibility of multiple hidden sectors to accommodate a successful framework to explain the abundance of matter over anti-matter and study prospects of a viable dark matter candidate. The evolution of cosmological history and the baryon asymmetry is studied thoroughly. The baryon asymmetry and the dark matter relic abundance are checked in light of the current cosmological data and a potential parameter space is computed. The analysis conducted reveals that a viable mechanism

We investigate the existence of a new fermionic partner to the top quark in light of the current experimental searches conducted at the colliders. The model presented here is phenomenologically attractive because it is less constrained than the traditional decay modes of the top partner. It also has the potential to cancel the quadratic divergences in the Higgs mass. In addition to the new fermion, the model contains another scalar particle that mediates interactions with the Standard Model light quarks. This new fermion is studied by using various recasted searches for previously unexplored decay channels(T −→ cη , T −→ tη , where η −→ uu). We thoroughly study the phenomenology of such model. From experimental searches, we find that current LHC searches exclude the top partner mass (new fermion) of about 350 GeV (and some other points in the m T m η -plane) for its decay to light quarks. On the other hand, when the top partner decays to a top and two light quarks, the lower limit obtained on the top partner mass can be as large as 650 GeV.iii Statement of OriginalityTo the best of my knowledge, the content of this thesis is my own work and all the assistance received in preparing this thesis and the sources used have been properly acknowledged.v