In the classic type I seesaw mechanism with very heavy right-handed (RH) neutrinos, it is possible to account for dark matter via RH neutrino portal couplings to a feebly interacting massive particle (FIMP) dark sector. However, for large RH neutrino masses, gravity can play an important role. We study the interplay between the neutrino portal through the right-handed neutrinos and the gravity portal through the massless spin-2 graviton in producing dark matter particles in the early universe. As a concrete example, we consider the minimal and realistic Littlest Seesaw model with two RH neutrinos, augmented with a dark scalar and a dark fermion charged under a global $U(1)_D$ dark symmetry. In the model, the usual seesaw neutrino Yukawa couplings and the right-handed neutrino masses (the lightest being about $5\times 10^{10}$ GeV) are fixed by neutrino oscillations data and leptogenesis. Hence, we explore the parameter space of the two RH neutrino portal couplings, the two dark particle masses and the reheating temperature of the universe, where the correct dark matter relic abundance is achieved through the freeze-in mechanism. In particular, we highlight which class of processes dominate the dark matter production. We find that, despite the presence of the gravity portal, the dark matter production relies on the usual seesaw neutrino Yukawa coupling in some regions of the parameter space, so realising a direct link between dark matter and neutrino phenomenology. Finally, we report the threshold values for the neutrino portal couplings below which the neutrino portal is irrelevant and the Planckian Interacting Dark Matter paradigm is preserved.

中文翻译：

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FIMP 暗物质中微子和重力门户之间的相互作用

在具有非常重的右手 (RH) 中微子的经典 I 型跷跷板机制中，可以通过 RH 中微子门户耦合到微弱相互作用的大质量粒子 (FIMP) 暗扇区来解释暗物质。然而，对于较大的 RH 中微子质量，重力可以发挥重要作用。我们研究了通过右手中微子的中微子入口和通过无质量自旋 2 引力子的重力入口之间的相互作用，在早期宇宙中产生暗物质粒子。作为一个具体的例子，我们考虑最小和现实的 Littlest Seesaw 模型，它有两个 RH 中微子，增加了一个暗标量和一个在全局 $U(1)_D$ 暗对称下充电的暗费米子。在模型中，通常的跷跷板中微子汤川耦合和右手中微子质量（最轻的大约是 $5\times 10^{10}$ GeV）由中微子振荡数据和轻质发生确定。因此，我们探索了两个 RH 中微子门户耦合、两个暗粒子质量和宇宙的再加热温度的参数空间，其中通过冻结机制实现了正确的暗物质遗迹丰度。我们特别强调了哪类过程在暗物质生产中占主导地位。我们发现，尽管存在重力入口，但暗物质的产生依赖于参数空间某些区域中通常的跷跷板中微子汤川耦合，因此实现了暗物质与中微子现象学之间的直接联系。最后，