Neff in the Standard Model at NLO is 3.043

The effective number of relativistic neutrino species is a fundamental probe of the early Universe, and its measurement represents a key constraint on many scenarios beyond the Standard Model of Particle Physics. In light of this, an accurate prediction of Neff in the Standard Model is of pivotal importance. In this work, we consider the last ingredient needed to accurately calculate NSM temperature QED corrections to e+e- <-> nu nu over bar interaction rates during neutrino decoupling at temperatures around T similar to MeV. We find that this effect leads to a reduction of -0.0007 in NSM order QED correction to the interaction rates, together with finite-temperature QED corrections to the electromagnetic density of the plasma, and the effect of neutrino oscillations, implies that NSM with a theoretical uncertainty that is much smaller than any projected observational sensitivity.