要旨
In this thesis, a new production mechanism for sterile neutrino dark matter is proposed, which, in contrast to previous research, relies neither on the oscillations between active and sterile neutrinos, nor on the decay of heavier additional degrees of freedom, nor on new exotic neutrino interactions beyond the Yukawa coupling to the Standard Model. Instead, we generate the abundance of sterile neutrinos by decoupling from thermal equilibrium, as is typical for WIMPs. This type of production mechanism is usually not viable for sterile neutrinos, because the longevity requirement mandates that the neutrino Yukawa coupling be very tiny, which prevents the dark matter neutrinos from reaching thermal equilibrium. We resolve this conflict by invoking varying Yukawa couplings, going from sizeable values at early times, thus enabling the sterile neutrinos to thermalize, and then becoming suppressed during a phase transition, thereby forcing the sterile neutrinos to decouple and stay quasi-stable thereafter. We formulate an implementation of varying Yukawa couplings based on a Froggatt-Nielsen model, where the vacuum expectation value of the flavon changes during a phase transition, thereby dynamically driving the suppression of the Yukawa couplings and inducing the decoupling of the sterile neutrinos. We show that our mechanism successfully generates 100\% of the observed dark matter in the form of sterile neutrinos with masses in the keV range. The necessary phase transition is provided by the spontaneous breaking of the electroweak symmetry. Furthermore, the active neutrino oscillation parameters are reproduced and simultaneously the flavour hierarchy in the lepton sector is alleviated.
