Relic neutrino freeze-out: Dependence on natural constants

Analysis of cosmic microwave background radiation fluctuations favors an effective number of neutrinos, N-v > 3. This motivates a reinvestigation of the neutrino freeze-out process. Here we characterize the dependence of N-v on the Standard Model (SM) parameters that govern neutrino freeze-out. We show that N-v depends on a combination eta of several natural constants characterizing the relative strength of weak interaction processes in the early Universe and on the Weinberg angle sin(2) theta w. We determine numerically the dependence N-v(eta, sin(2) theta 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. (C) 2014 The Authors. Published by Elsevier B.V.