Low-scale new physics in dark sectors

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 $\sim \! 10 \TeV$ in the scotogenic model, while for singlet scalar assisted leptogenesis we can even reach scales below $1 \TeV$. 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 $t \bar{t}$, 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.