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 νR's to decouple much earlier, at a higher temperature, than their left-handed counterparts. If the νR's decouple during the quark-hadron crossover transition, they are considerably cooler than the νL's and contribute less than 3 extra "equivalent neutrinos" to the early Universe energy density. For decoupling in this transition region, the 3 νR generate ∆Nν = 3(Tν R /Tν 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.

Minimal model of Majoronic dark radiation and dark matter

“…Light sterile neutrinos were considered in [11]. In addition, righthanded neutrinos with milliweak interactions as DR were attended to in [12]. Contribution to N eff from axion-like particles was mentioned in [13].…”

Nonthermal cosmic neutrino background

“…Examples include scenarios with a gauged B − L symmetry which is broken at the TeV scale [6][7][8]. Here one obtains a RH neutrino background with an almost thermal spectrum that is, at the time of BBN, colder than the SM particles.…”

Multicomponent Dark Matter in Radiative Seesaw Models

“…The most important fact is that the entropy per comoving volume is conserved, so that sa 3 is constant, where s is the entropy density, and a is the scale factor. The effective number N eff follows from ρ r (T γ 0 ) = (π 2 /15)(1 + (7/8)(4/11) 4/3 N eff ) T 4 γ 0 and is given by Kolb and Turner [125], Steigman [140], Anchordoqui and Goldberg [141], Steigman et al [142], Anderhalden et al [143,145], Anchordoqui et al [144], and Weinberg [146] …”