Reconciling BICEP2 and Planck results with right-handed Dirac neutrinos in the fundamental representation of grand unified E₆

Abstract

The tensor-to-scalar ratio (r = 0.20^+0.07_-0.05) inferred from the excess B-mode power observed by the Background Imaging of Cosmic Extragalactic Polarization (BICEP2) experiment is almost twice as large as the 95% CL upper limits derived from temperature measurements of the WMAP (r < 0.13) and Planck (r < 0.11) space missions. Very recently, it was suggested that additional relativistic degrees of freedom beyond the three active neutrinos and photons can help to relieve this tension: the data favor an effective number of light neutrino species N_eff = 3.86±0.25. Since the BICEP2 ratio implies the energy scale of inflation (V_*^1/4 ~ 2 × 10¹⁶ GeV) is comparable to the grand unification scale, in this paper we investigate whether we can accommodate the required N_eff with three right-handed (partners of the left-handed standard model) neutrinos living in the fundamental representation of a grand unified exceptional E₆ group. We show that the superweak interactions of these Dirac states (through their coupling to a TeV-scale Z' gauge boson) lead to decoupling of right-handed neutrino just above the QCD cross over transition: 175 MeVlesssimT_νR^declesssim250 MeV. For decoupling in this transition region, the contribution of the three right-handed neutrinos to N_eff is suppressed by heating of the left-handed neutrinos (and photons). Consistency (within 1σ) 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 with future data from LHC14.