Majorana Neutrino Magnetic Moment and Neutrino Decoupling in Big Bang Nucleosynthesis

TL;DR

This paper investigates how transition magnetic moments of Majorana neutrinos influence their decoupling in the early universe and the subsequent effects on Big Bang Nucleosynthesis, finding current constraints limit observable impacts.

Contribution

It provides a detailed calculation of how Majorana neutrino magnetic moments affect decoupling temperatures and primordial element abundances in cosmology.

Findings

Magnetic moments around 10^{-10} Bohr magnetons significantly alter nucleosynthesis predictions.

Recent Borexino data constrains Majorana moments to below 10^{-11} Bohr magnetons.

Changes in cosmological parameters due to magnetic moments are below upcoming observational sensitivities.

Abstract

We examine the physics of the early universe when Majorana neutrinos (electron neutrino, muon neutrino, tau neutrino) possess transition magnetic moments. These extra couplings beyond the usual weak interaction couplings alter the way neutrinos decouple from the plasma of electrons/positrons and photons. We calculate how transition magnetic moment couplings modify neutrino decoupling temperatures, and then use a full weak, strong, and electromagnetic reaction network to compute corresponding changes in Big Bang Nucleosynthesis abundance yields. We find that light element abundances and other cosmological parameters are sensitive to magnetic couplings on the order of 10^{-10} Bohr magnetons. Given the recent analysis of sub-MeV Borexino data which constrains Majorana moments to the order of 10^{-11} Bohr magnetons or less, we find that changes in cosmological parameters from magnetic contributions to neutrino decoupling temperatures are below the level of upcoming precision observations.