Dark radiation confronting LHC in Z′ models

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 E6Z′ models to account for the present evidence of dark radiation with the already existing constraints from searches for new neutral gauge bosons at LHC7.