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#### Seesaw neutrino dark matter by freeze-out

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##### Citation

Jaramillo, C., Lindner, M., & Rodejohann, W. (2021). Seesaw neutrino dark matter
by freeze-out.* Journal of Cosmology and Astroparticle Physics,* *2021*(04):
023. doi:10.1088/1475-7516/2021/04/023.

Cite as: https://hdl.handle.net/21.11116/0000-0009-034C-F

##### 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.

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.