If neutrinos are Dirac particles the existence of light right-handed neutrinos $\nu_{R}$ is implied. Those would contribute to the effective number of relativistic neutrino species $N_{{\rm eff}}$ in the early Universe. With pure standard model interactions, the contribution is negligibly small. In the presence of new interactions, however, the contribution could be significantly enhanced. We consider the most general effective four-fermion interactions for neutrinos (scalar, pseudo-scalar, vector, axial-vector and tensor), and compute the contribution of right-handed neutrinos to $N_{{\rm eff}}$. Taking the Planck 2018 measurement of $N_{{\rm eff}}$, strong constraints on the effective four-fermion coupling are obtained, corresponding to interaction strengths of $10^{-5}\sim10^{-3}$ in units of the Fermi constant. This translates in new physics scales of up to 43 TeV and higher. Future experiments such as CMB-S4 can probe or exclude the existence of effective 4-neutrino operators for Dirac neutrinos. Ways to avoid this conclusion are discussed.

中文翻译：

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狄拉克中微子和内夫

如果中微子是狄拉克粒子，则暗示存在轻右手中微子 $\nu_{R}$。这些将有助于形成早期宇宙中相对论中微子物种 $N_{{\rm eff}}$ 的有效数量。对于纯标准模型交互，贡献可以忽略不计。然而，在存在新的交互作用的情况下，贡献可能会显着增强。我们考虑了中微子（标量、伪标量、向量、轴向量和张量）的最普遍有效的四费米子相互作用，并计算右手中微子对 $N_{{\rm eff}}$ 的贡献。取普朗克 2018 年对 $N_{{\rm eff}}$ 的测量，获得了对有效四费米子耦合的强约束，对应于单位为 $10^{-5}\sim10^{-3}$ 的相互作用强度费米常数。这转化为高达 43 TeV 或更高的新物理尺度。未来的实验如 CMB-S4 可以探测或排除狄拉克中微子的有效 4-中微子算子的存在。讨论了避免这一结论的方法。