Seesaw neutrino dark matter by freeze-out

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.

Reviving keV sterile Neutrino Dark Matter

“…Indeed, the decay of sterile neutrinos with a non-vanishing Yukawa coupling is unavoidable and has been searched for with X-Ray surveys, placing strict upper bounds on the mixing angle between sterile and active neutrinos, and equivalently, on the Yukawa coupling. As we pointed out previously in [41], this can be solved by varying Yukawa couplings: if, by some mechanism, the Yukawa couplings were large at early times but became very small at later times, then the sterile neutrinos could have been in thermal equilibrium in the early Universe, then decouple and stay stable on cosmological timescales thereafter.…”

“…To achieve the thermal freeze-out of sterile neutrinos, we propose to implement varying Yukawa couplings in the following way, resembling [27,41]. Since the scalar potential of the theory will include both the Higgs φ and the flavon Θ, the true minimum of the scalar potential will be located at a point in the (φ, Θ)-field space.…”

Sterile neutrino dark matter: relativistic freeze-out

“…In this work, we study the possibility that the lightest sterile neutrino reaches thermal equilibrium with the singlet scalar and subsequently freezes out. Thermal neutrino freeze-out has been shown to be a viable option for obtaining the correct DM relic abundance in various models (see e.g., [15,16,17,18]).…”

Dark matter transient annihilations in the early Universe

“…There have been prior studies on the effects cosmological evolution have on DM or scalar mediator mass, stability, interaction couplings, and annihilation channels [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47]. In our scenario, we, for the first time, focus on the vector mediator whose mass is significantly affected during the freeze-out.…”