Relic neutrino freeze-out: Dependence on natural constants

Analysis of cosmic microwave background radiation fluctuations favors an effective number of neutrinos, N_ν>3. This motivates a reinvestigation of the neutrino freeze-out process. Here we characterize the dependence of N_ν on the Standard Model (SM) parameters that govern neutrino freeze-out. We show that N_ν depends on a combination η of several natural constants characterizing the relative strength of weak interaction processes in the early Universe and on the Weinberg angle sin² θ_W. We determine numerically the dependence N_ν(η, sin² θ_W) and discuss these results. The extensive numerical computations are made possible by two novel numerical procedures: a spectral method Boltzmann equation solver adapted to allow for strong reheating and emergent chemical non-equilibrium, and a method to evaluate Boltzmann equation collision integrals that generates a smooth integrand.