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Abstract

We investigate whether right-handed neutrinos can play the role of the dark
matter of the Universe and be generated by the freeze-out production mechanism.
In the standard picture, the requirement of a long lifetime of the right-handed
neutrinos implies a small neutrino Yukawa coupling. As a consequence, they
never reach thermal equilibrium, thus prohibiting production by freeze-out. We
note that this limitation is alleviated if the neutrino Yukawa coupling is
large enough in the early Universe to thermalize the sterile neutrinos, and
then becomes tiny at a certain moment, which makes them drop out of
equilibrium. As a concrete example realization of this framework, we consider a
Froggatt-Nielsen model supplemented by an additional scalar field which obeys a
global symmetry (not the flavour symmetry). Initially, the vacuum expectation
value of the flavon is such, that the effective neutrino Yukawa coupling is
large and unsuppressed, keeping them in thermal equilibrium. At some point the
new scalar also gets a vacuum expectation value that breaks the symmetry. This
may occur in such a way that the vev of the flavon is shifted to a new
(smaller) value. In that case, the Yukawa coupling is reduced such that the
sterile neutrinos are rendered stable on cosmological time scales. We show that
this mechanism works for a wide range of sterile neutrino masses.