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

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 ν_R's to decouple much earlier, at a higher temperature, than their left-handed counterparts. If the ν_R's decouple during the quark-hadron crossover transition, they are considerably cooler than the ν_L's and contribute less than 3 extra "equivalent neutrinos" to the early Universe energy density. For decoupling in this transition region, the 3 ν_R generate ΔN_ν= 3(T_{ν_R}/T_{ν_ 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.