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The tensor-to-scalar ratio (r = 0.20^{+0.07}_{-0.05}) inferred from theexcess B-mode power observed by the Background Imaging of Cosmic ExtragalacticPolarization (BICEP2) experiment is almost twice as large as the 95% CL upperlimits derived from temperature measurements of the WMAP (r<0.13) and Planck(r<0.11) space missions. Very recently, it was suggested that additionalrelativistic degrees of freedom beyond the three active neutrinos and photonscan help to relieve this tension: the data favor an effective number of lightneutrino species N_{eff} = 3.86 \pm 0.25. Since the BICEP2 ratio implies theenergy scale of inflation (V_*^{1/4} \sim 2 \times 10^{16} GeV) is comparableto the grand unification scale, in this paper we investigate whether we canaccommodate the required N_{eff} with three right-handed (partners of theleft-handed standard model) neutrinos living in the fundamental representationof a grand unified exceptional E_6 group. We show that the superweakinteractions of these Dirac states (through their coupling to a TeV-scale Z'gauge boson) lead to decoupling of right-handed neutrino just above the QCDcross over transition: 175 MeV < T_{\nu_R}^{dec} < 250 MeV. For decoupling inthis transition region, the contribution of the three right-handed neutrinos toN_{eff} is suppressed by heating of the left-handed neutrinos (and photons).Consistency (within 1\sigma) with the favored N_{eff} is achieved for 4.5 TeV <M_{Z'} < 7.5 TeV. The model is fully predictive and can be confronted withfuture data from LHC14.

Reconciling BICEP2 and Planck results with right-handed Dirac neutrinos in the fundamental representation of grand unified E_6