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.

我们研究右旋中微子是否可以扮演宇宙暗物质的角色并由冻结产生机制产生。在标准图片中，右旋中微子的长寿命要求意味着小中微子汤川耦合。因此，它们永远不会达到热平衡，从而禁止通过冻结生产。我们注意到，如果中微子 Yukawa 耦合在早期宇宙中足够大以热化无菌中微子，然后在某个时刻变得很小，从而使它们失去平衡，则这种限制会得到缓解。作为该框架的具体示例实现，我们考虑了一个 Froggatt-Nielsen 模型，该模型由一个额外的标量场补充，该标量场遵循全局对称性（不是风味对称性）。原来，黄酮的真空期望值是这样的，即有效的中微子 Yukawa 耦合很大且不受抑制，使它们保持热平衡。在某些时候，新标量也会获得打破对称性的真空期望值。这可能会以这样一种方式发生，即黄酮的 vev 转移到一个新的（较小的）值。在这种情况下，汤川耦合被减少，使得无菌中微子在宇宙学时间尺度上变得稳定。我们表明，这种机制适用于广泛的无菌中微子质量。在这种情况下，汤川耦合被减少，使得无菌中微子在宇宙学时间尺度上变得稳定。我们表明，这种机制适用于广泛的无菌中微子质量。在这种情况下，汤川耦合被减少，使得无菌中微子在宇宙学时间尺度上变得稳定。我们表明，这种机制适用于广泛的无菌中微子质量。