Strong cosmological constraints on the neutrino magnetic moment

Abstract

A sizable magnetic moment for neutrinos would be evidence of exotic physics. In the early Universe, left-handed neutrinos with a magnetic moment would interact with electromagnetic fields in the primordial plasma, flipping their helicity and producing a population of right-handed (RH) neutrinos. In this work, we present a new calculation of the production rate of RH neutrinos in a multi-component primordial plasma and quantify their contribution to the total energy density of relativistic species at early times, stressing the implications of the dependence on the initial time for production. We find that current cosmological data exclude values of the magnetic moment $\hat{I}¼\gtrsim 1.6\times 10^{-11}\hat{I}{¼}_B$, while future cosmological experiments will be able to probe non-thermal production of RH neutrinos, becoming competitive with stellar limits.

10 pages, 5 figures. Matches the version published on PRD. Changes: We added a more detailed discussion on current constraints and future sensitivities. We added two appendices, giving details on the freeze-in regime and discussing the impact of the different cosmological analyses, respectively