| Abstract [eng] |
n our universe, baryonic (visible) matter only comprises of the universe's energy content, while around 70% is attributed to the unknown dark energy. The rest is considered to be very weakly interacting particles, mostly interacting with matter through gravity and not interacting electromagnetically, known as dark matter. However, there are possible candidate particles, like sterile neutrinos, that could be dark matter. In the Standard Model of particle physics, neutrinos do not have a mass term. That is in contradiction with experiments, where, by observing neutrino oscillations, it was shown that neutrinos do have mass. In the Standard Model, particles acquire mass through the interaction with the Higgs field with their right-(left-)handed counterparts. However, in the Standard Model, the neutrinos are only left-handed. Which means that there should be right-handed neutrinos. These right-handed or sterile neutrinos are weakly interacting - they only allow left-handed neutrinos (the ones we know) to have mass. One possible extension to the Standard Model, called the Grimus-Neufeld model not only adds a heavy sterile neutrino, it also states that neutrinos are Majorana neutrinos and also adds a second Higgs doublet. Because the sterile neutrinos that come from the Grimus-Neufeld model are weakly interacting, mainly through the Higgs field and gravity, one can say that these particles could be dark matter. This is because we assume them to be heavy - heavier than the left-handed neutrinos and maybe heavier than other particles. And of course because they are so weakly interacting. In this work, we look over the possibility of sterile neutrinos being dark matter. We begin by presenting the basics of cosmology. Namely by showing how the Boltzmann and Einstein's equations are used to calculate the evolution of all particle densities, including dark matter. This is done in the case of non-thermal equilibrium. We then look over possible sterile neutrino generation methods: the Dodelson-Widrow or the thermal model and the heavy scalar decay model. For both of these models we derive the neutrino number density n_{S} formulas. In the heavy scalar decay, we derive two formulas for two periods of sterile neutrino production - early scalar production/decay and late scalar decay. Using these formulas, we calculate the density parameter of sterile neutrinos \varOmega_{S} for both cases and a third combined case and show at what sterile neutrino mass does the sterile neutrino density parameter resemble that of the dark matter density parameter. For the heavy scalar decay we also look at how the additional parameters that come from this model, like the coupling between the scalar and neutrino fields and the scalar mass affect the mass of the sterile neutrinos. Finally, we look at the results and the possible implications of these models. We compare the two models between themselves. And lastly, we list the possible shortcomings of the models. |
