# Observing Dirac neutrinos in the cosmic microwave background

## TL;DR

Future CMB experiments could detect signatures of Dirac neutrinos through precise measurements of $N_ ext{eff}$, providing insights into new physics interactions and the early Universe's thermal history.

## Contribution

This paper analyzes how upcoming CMB experiments can constrain Dirac neutrino models with additional interactions, linking cosmological observations to particle physics.

## Key findings

- CMB-S4 can significantly improve constraints on Dirac neutrino interactions.
- Models like gauged $U(1)_{B-L}$ and neutrinophilic two-Higgs-doublet are testable with future CMB data.
- Comparison shows CMB experiments complement collider searches for new neutrino interactions.

## Abstract

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.