Observing Dirac neutrinos in the cosmic microwave background

Kevork N Abazajian,Julian Heeck

doi:10.1103/physrevd.100.075027

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.

Highlights

The sensitivity of anisotropies in the cosmic microwave background (CMB) to extra radiation density like that in the form of effective extra numbers of neutrinos Neff has been known for some time [1]
Upcoming limits from the CMB and large-scale structure on extra radiation from the early Universe are entering a qualitatively new regime, with sensitivity to particle species that have decoupled from equilibrium at very early times and high energy scales
We argued that one of the best motivations for such light particles comes from the observation of neutrino oscillations

Summary

INTRODUCTION

The sensitivity of anisotropies in the cosmic microwave background (CMB) to extra radiation density like that in the form of effective extra numbers of neutrinos Neff has been known for some time [1]. While it is well known that just SM þ Dirac ν does not put νR in equilibrium due to the tiny Yukawa couplings mν=hHi ≲ 10−11 [9,10], one often expects additional interactions for νR in order to explain the smallness of neutrino masses, to generate the observed matter–antimatter asymmetry of our Universe, and to protect the Dirac nature from quantum gravity, as we will highlight below All of these new νR interactions will face strong constraints from CMB-S4 that will make it difficult to see the mediator particles in any other experiment, in particular at the LHC.

OBSERVING Neff

IMPACT ON DIRAC NEUTRINO MODELS

Neutrinophilic 2HDM and other mass models

Leptogenesis

CONCLUSION