A theory of extra radiation in the Universe

TL;DR

The paper analyzes the existence of extra radiation in the Universe, encoded as $\Delta N_{\rm eff}$, motivated by $Y_p$, CMB, and LSS observations. It tests several production histories for relativistic species in thermal equilibrium and concentrates on the case of a single light chiral fermion with TeV-scale suppressed couplings that decouples after the QCD transition, which can yield $\Delta N_{\rm eff} \sim 1$ and be detectable at the LHC. A concrete $E_6$-inspired GUT realization provides such a fermion as a singlet $\psi_1$ per generation and predicts additional TeV-scale exotics, offering collider signatures. Planck-level measurements of $\Delta N_{\rm eff}$ and LHC searches for a TeV-scale gauge boson $A_H$ or long-lived colored states together enable a decisive test of this scenario.

Abstract

Recent cosmological observations, such as the measurement of the primordial 4He abundance, CMB, and large scale structure, give preference to the existence of extra radiation component, Delta N_nu > 0. The extra radiation may be accounted for by particles which were in thermal equilibrium and decoupled before the big bang nucleosynthesis. Broadly speaking, there are two possibilities: 1) there are about 10 particles which have very weak couplings to the standard model particles and decoupled much before the QCD phase transition; 2) there is one or a few light particles with a reasonably strong coupling to the plasma and it decouples after the QCD phase transition. Focusing on the latter case, we find that a light chiral fermion is a suitable candidate, which evades astrophysical constraints. Interestingly, such a scenario may be confirmed at the LHC. As a concrete example, we show that such a light fermion naturally appears in the E_6-inspired GUT.