Neutrino Magnetic Moments Meet Precision $N_{\rm eff}$ Measurements

TL;DR

This paper calculates the impact of neutrino magnetic moments on early universe physics, deriving new constraints from cosmological measurements that surpass some laboratory bounds.

Contribution

It provides a detailed computation of neutrino chirality-flipping rates considering finite temperature effects, leading to improved bounds on neutrino magnetic moments from cosmological data.

Findings

Neutrino magnetic moments above 2.7×10⁻¹² μ_B are excluded by current cosmological data.

The derived bounds are stronger than recent laboratory experiments.

The limits are comparable to stellar cooling constraints.

Abstract

In the early universe, Dirac neutrino magnetic moments due to their chirality-flipping nature could lead to thermal production of right-handed neutrinos, which would make a significant contribution to the effective neutrino number, $N_{\rm eff}$. We present in this paper a dedicated computation of the neutrino chirality-flipping rate in the thermal plasma. With a careful and consistent treatment of soft scattering and the plasmon effect in finite temperature field theories, we find that neutrino magnetic moments above $2.7\times 10^{-12}\mu_B$ have been excluded by current CMB and BBN measurements of $N_{\rm eff}$, assuming flavor-universal and diagonal magnetic moments for all three generation of neutrinos. This limit is stronger than the latest bounds from XENONnT and LUX-ZEPLIN experiments, and comparable with those from stellar cooling considerations.