Analysis of cosmic microwave background radiation fluctuations favors an effective number of neutrinos , N _ { \nu } > 3 .
This motivates a reinvestigation of the neutrino freeze-out process .
Here we characterize the dependence of N _ { \nu } on the Standard Model ( SM ) parameters that govern neutrino freeze-out .
We show that N _ { \nu } 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 _ { \nu } ( \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 .