The gauge-extended $U(1{)}_C\times SU(2{)}_L\times U(1{)}_{I_R}\times U(1{)}_L$ model elevates the global symmetries of the standard model (baryon number $B$ and lepton number $L$) to local gauge symmetries. The $U(1{)}_L$ symmetry leads to three superweakly interacting right-handed neutrinos. This also renders a $B-L$ symmetry nonanomalous. The superweak interactions of these Dirac states permit ${\nu }_R$ decoupling just above the QCD phase transition: $175\lesssim T_{{\nu }_R}^{\mathrm{dec}}/\mathrm{MeV}\lesssim 250$. In this transitional region, the residual temperature ratio between ${\nu }_L$ and ${\nu }_R$ generates extra relativistic degrees of freedom at BBN and at the CMB epochs. Consistency with both WMAP 7-year data and recent estimates of the primordial ${}^4\mathrm{He}$ mass fraction is achieved for $3<M_{B-L}/\mathrm{TeV}<6$. The model is fully predictive, and can be confronted with dijet and dilepton data from LHC7 and, eventually, LHC14.

• Received 1 December 2011

DOI:https://doi.org/10.1103/PhysRevLett.108.081805