## Abstract

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 sin^{2} θ_{W}. We determine numerically the dependence N_{ν}(η, sin^{2} 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.

Original language | English (US) |
---|---|

Pages (from-to) | 481-517 |

Number of pages | 37 |

Journal | Nuclear Physics B |

Volume | 890 |

DOIs | |

State | Published - Jan 1 2015 |

## ASJC Scopus subject areas

- Nuclear and High Energy Physics