Observing Dirac neutrinos in the cosmic microwave background

Planned CMB Stage IV experiments have the potential to measure the effective number of relativistic degrees of freedom in the early Universe, N₄₅₅, with percent-level accuracy. This probes new thermalized light particles and also constrains possible new-physics interactions of Dirac neutrinos. Many Dirac-neutrino models that aim to address the Dirac stability, the smallness of neutrino masses or the matter--antimatter asymmetry of our Universe endow the right-handed chirality partners ₑ with additional interactions that can thermalize them. Unless the reheating temperature of our Universe was low, this leads to testable deviations in N₄₅₅. We discuss well-motivated models for ₑ interactions such as gauged U (1) ₁-₋ and the neutrinophilic two-Higgs-doublet model, and compare the sensitivity of SPT-3G, Simons Observatory, and CMB-S4 to other experiments, in particular the LHC.