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Abstract:
In this thesis we investigate two particular examples of phenomena that require
introducing new physics beyond the standard model, namely the baryon asymmetry
of the universe and dark matter. As the corresponding newly introduced fields or
particles have eluded detection so far, they are usually associated with so-called dark
sectors. Our focus throughout this work is on low-scale realizations of mechanisms
explaining these phenomena, with low scale either referring to a comparison to the
standard scenarios of the mechanisms, or the mass scales of the fields or particles
involved. One well-established mechanism to explain the baryon asymmetry of the
universe is leptogenesis. We study the possibility to realize low-scale leptogenesis in
both the scotogenic and the singlet scalar assisted model by employing analytical and
semi-analytical methods, with a focus on understanding the important ingredients.
Our parameter scans show that we are able to recreate the baryon asymmetry in the
universe via leptogenesis for right-handed neutrino masses of as low as ~10TeV in
the scotogenic model, while for singlet scalar assisted leptogenesis we can even reach
scales below 1TeV. Importantly, both of these results are achieved without a strong
degeneracy of right-handed neutrino masses. In our study of dark matter, we first
analyze and compare the LHC signatures of two benchmark models given by the two
Higgs doublet model with an additional scalar or pseudoscalar. To do so, we study
their tt¯, mono-Z and mono-h signatures and derive limits from current experimental
searches at the LHC. Furthermore, we also look at the reach of the mono-Z channel for
future LHC upgrades and comment on the possibility to distinguish between the two
models in case of a signal detection. Finally, we analyze a possibility to explain dark
matter without using the standard particle dark matter picture via (pseudo)scalar and
vector dark matter from non-minimal curvature couplings. With the misalignment and
stochastic scenario, we investigate two different options how the dark matter could
be created during the period of inflation. In the misalignment scenario we find that
the parameter space substantially opens up due to the non-minimal coupling, whereas
in the stochastic scenario any non-minimal coupling is tightly constrained for the
mechanism to work while not violating isocurvature constraints. We conclude this
thesis with a recapitulation of our main results and an outlook to further research.
