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.

Figures (3)

• Figure 1: Comparing the BBN predictions of $N_{\rm eff}$ and $\Omega_{\rm B} h^2$ with those from various CMB determinations: BBN D + $^4$He (red filled triangle) Steigman:2012ve, BBN D + WMAP7 Komatsu:2010fb (red open triangle), WMAP7 Komatsu:2010fb (blue filled square), ACT Dunkley:2010ge (green filled pentagon), SPT Keisler:2011aw (purple filled circle), SPT + Clusters Hou:2011ec (purple open circle). Taken from Ref. Steigman:2012ve.
• Figure 2: Relation between $\Delta N_\nu \ {\it vs.} \ T_{\nu_R}^{\rm dec}$.
• Figure 3: The green cross-hatched areas show the region allowed from decoupling requirements to accommodate BBN and CMB eras. Each of the horizontal lines refers to a particular model: from the top the $E_6$$Z'_\chi$, a D-brane model in which $Z'$ is mostly $B-L$, a D-brane model in which $Z'$ couples mostly to the third component of a right-handed isospin, a D-brane model with TeV-scale strings, the $E_6$$Z'_\psi$. Termination of the lines on the left reflects the LHC experimental limits on the mass of the gauge boson. The left and right figures show the condition on decoupling for loss of chemical and thermal equilibrium, respectively.