  Planned CMB Stage IV experiments have the potential to measure the effective
number of relativistic degrees of freedom in the early Universe,
$N_\text{eff}$, 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--anti-matter asymmetry of our
Universe endow the right-handed chirality partners $\nu_R$ with additional
interactions that can thermalize them. Unless the reheating temperature of our
Universe was low, this leads to testable deviations in $N_\text{eff}$. We
discuss well-motivated models for $\nu_R$ 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.
