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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.
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We study the stochastic background of gravitational waves which accompany the sudden freeze-out of dark matter triggered by a cosmological first order phase transition that endows dark matter with mass. We consider models that produce the measured dark matter relic abundance via (1) bubble filtering, and (2) inflation and reheating, and show that gravitational waves from these mechanisms are detectable at future interferometers.
We study a class of general U$(1)^prime$ models to explain the observed dark matter relic abundance and light neutrino masses. The model contains three right handed neutrinos and three gauge singlet Majorana fermions to generate the light neutrino mass via the inverse seesaw mechanism. We assign one pair of degenerate sterile neutrinos to be the dark matter candidate whose relic density is generated by the freeze-in mechanism. We consider different regimes of the masses of the dark matter particle and the ${Z^prime}$ gauge boson. The production of the dark matter can occur at different reheating temperatures in various scenarios depending on the masses of the ${Z^prime}$ boson and the dark matter candidate. We also note that if the mass of the sterile neutrino dark matter is $gtrsim 1 rm{MeV}$ and if the $Z^prime$ is heavier than the dark matter, the decay of the dark matter candidate into positrons can explain the long standing puzzle of the galactic $511rm{keV}$ line in the Milky Way center observed by the INTEGRAL satellite. We constrain the model parameters from the dark matter analysis, vacuum stability and the collider searches of heavy ${Z^prime}$ at the LHC. For the case with light $Z^prime$, we also compare how far the parameter space allowed from dark matter relic density can be probed by the future lifetime frontier experiments SHiP and FASERs in the special case of $U(1)_{B-L}$ model.
We investigate a minimal neutrino portal dark matter (DM) model where a right-handed neutrino both generates the observed neutrino masses and mediates between the SM and the dark sector, which consists of a fermion and a boson. In contrast to earlier work, we explore regions of the parameter space where DM is produced via freeze-in instead of freeze-out motivated by the small neutrino Yukawa couplings in case of $mathcal{O} left( mathrm{TeV} right)$ heavy neutrinos. For a non-resonant production of DM, its energy density is independent of the DM mass. Assuming a democratic coupling structure we find $M_N approx 10 , mathrm{TeV}$. For the resonant production of DM, we find that it can be produced via freeze-in or freeze-out even with couplings of $mathcal{O} left( 10^{-5} right)$. However, the measurement of the Lyman-$alpha$ forest rules out the feeble coupled freeze-out case completely, while the resonant freeze-in production is only viable for $m_{DM} gtrsim 3 , mathring{keV}$.
It is quite conceivable that dark matter freeze-out occurred during an early period of matter domination, in which case the evolution and relic abundance differ from standard freeze-out calculations which assume a radiation dominated universe. Here we re-examine the classic models in which dark matter interactions with the Standard Model are mediated via either the Higgs or $Z$ boson in the context of matter dominated freeze-out. We highlight that while these classic models are largely excluded by searches in the radiation dominated case, matter dominated freeze-out can relax these limits and thus revive the Higgs and $Z$ portals. Additionally, we discuss the distinctions between matter dominated freeze-out and decoupling during the transition from matter domination to radiation domination, and we comment on the parameter regimes which lead to non-negligible dark matter production during this transition.
We consider dark matter (DM) with very weak couplings to the standard model (SM), such that its self-annihilation cross section is much smaller than the canonical one, $langlesigma vrangle_{chichi} ll 10^{-26}mathrm{cm}^3/mathrm{s}$. In this case DM self-annihilation is negligible for the dynamics of freeze-out and DM dilution is solely driven by efficient annihilation of heavier accompanying dark sector particles provided that DM maintains chemical equilibrium with the dark sector. This chemical equilibrium is established by conversion processes which require much smaller couplings to be efficient than annihilation. The chemical decoupling of DM from the SM can either be initiated by the freeze-out of annihilation, resembling a co-annihilation scenario, or of conversion processes, leading to the scenario of conversion-driven freeze-out. We focus on the latter and discuss its distinct phenomenology.
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