Right-Handed Neutrinos as the Dark Radiation: Status and Forecasts for the LHC

TL;DR

This paper explores how right-handed neutrinos could account for dark radiation, analyzing their decoupling in the early universe and predicting potential signals detectable at the LHC.

Contribution

It provides a model-independent analysis of right-handed neutrinos as dark radiation and links their decoupling to observable collider signatures.

Findings

Right-handed neutrinos decouple during the quark-hadron transition, contributing less than 3 extra neutrinos.

Decoupling temperature affects the effective number of neutrinos, consistent with cosmological constraints.

Allowed parameter space for Z' bosons is within LHC discovery range.

Abstract

Precision data from cosmology (probing the CMB decoupling epoch) and light-element abundances (probing the BBN epoch) have hinted at the presence of extra relativistic degrees of freedom, the so-called "dark radiation." We present a model independent study to account for the dark radiation by means of the right-handed partners of the three, left-handed, standard model neutrinos. We show that milli-weak interactions of these Dirac states (through their coupling to a TeV-scale Z' gauge boson) may allow the \nu_R's to decouple much earlier, at a higher temperature, than their left-handed counterparts. If the \nu_R's decouple during the quark-hadron crossover transition, they are considerably cooler than the \nu_L's and contribute less than 3 extra "equivalent neutrinos" to the early Universe energy density. For decoupling in this transition region, the 3 \nu_R generate \Delta N_\nu = 3(T_{\nu_R}/T_{\nu_ L})^4 < 3, extra relativistic degrees of freedom at BBN and at the CMB epochs. Consistency with present constraints on dark radiation permits us to identify the allowed region in the parameter space of Z' masses and couplings. Remarkably, the allowed region is within the range of discovery of LHC14.