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, Neff, 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 νR with additional interactions that can thermalize them. Unless the reheating temperature of our Universe was low, this leads to testable deviations in Neff. We discuss well-motivated models for νR interactions such as gauged U(1)_B-L 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.