Dark Radiation Confronting LHC in Z' Models

Abstract

Recent cosmological data favour additional relativistic degrees of freedom beyond the three active neutrinos and photons, often referred to as "dark radiation". Extensions of the SM involving TeV-scale Z' gauge bosons generically contain superweakly interacting light right-handed neutrinos which can constitute this dark radiation. In this letter we confront the requirement on the parameters of the E6 Z' models to account for the present evidence of dark radiation with the already existing constraints from searches for new neutral gauge bosons at LHC7.

Figures (4)

• Figure 1: 95% upper limits on the productino ratio $\sigma(pp \rightarrow Z') \times {\rm Br}(Z'\rightarrow l^+l)$ from the combined di-muon and di-electron searches in Ref.CMS as a function of the resonance mass. Also shown in the figure the predictions for E$_6$$Z'$ models for different values of the model parameter $\beta$ as labeled in the figure. The curves corresponds from left to right to the values of $\beta$ listed from top to bottom.
• Figure 2: Left:The effective number of degrees of freedom as a function of the temperature without including the contribution of the right-handed neutrinos. Right: Decoupling temperature of the right-handed neutrinos as a function of the $Z'$ mass for different values of the model parameter $\beta$ as labeled in the figure. The curves corresponds from left to right to the values of $\beta$ listed from top to bottom.
• Figure 3: The equivalent number of extra neutrinos $\Delta N_{\rm eff}$ due to 3 generation of massless $\nu_R$'s as a function of the $Z'$ mass for several values of the model parameter $\beta$ as labeled in the figure. The curves corresponds from left to right to the values of $\beta$ listed from top to bottom.
• Figure 4: Required values of $M_{Z'}$ for two illustrative 1$\sigma$ ranges of $N_{\rm eff}$, $N_{\rm eff} =4.35^{+1.4}_{-0.54}$ ($0.76\leq\Delta N_{\rm eff}\leq 2.7$) as obtained in the analysis of Ref. GonzalezGarcia:2010un (left panel) and for the lower range presently favoured by BBN nucleosynthesis $N_{\rm eff} =3.71^{+0.47}_{-0.45}$ ($0.21\leq\Delta N_{\rm eff}\leq 1.13$) (right panel). The solid red curve are the 95% CL lower bounds on $M_{Z'}$ from searches for $Z'$ resonance in di-lepton production at LHC4 in Ref.CMS.