Evolution of Primordial Neutrino Helicities in Astrophysical Magnetic Fields and Implications for Their Detection

Since decoupling in the early Universe in helicity states, primordial neutrinos propagating in astrophysical magnetic fields precess and undergo helicity changes. In view of various experimental bounds allowing a large magnetic moment of neutrinos, we estimate the helicity flipping for relic neutrinos in both cosmic and galactic magnetic fields. The flipping probability is sensitive both to the neutrino magnetic moment and the structure of the magnetic fields and is a potential probe of the fields. As we find, even a magnetic moment well below that suggested by XENON1T could significantly affect relic neutrino helicities and their detection rate via inverse tritium beta decay.

Figure 1

The coefficient $A_{\mathrm{eff}}$ vs mass of the lightest neutrino for Dirac and Majorana neutrinos, for the normal (NH) and inverted (IH) hierarchies. The dashed curves show the extreme case of complete helicity flip from left to right handed. The dash-dotted curve shows the result for Dirac NH neutrinos with $⟨{\theta }^2⟩$ given by the Milky Way estimate (7), with $B=10\mu \mathrm{G}$, ${\mathrm{\Lambda }}_g=1\mathrm{kpc}$, and ${\mu }_{\nu }=5\times {10}^{-14}{\mu }_B$. The present neutrino temperature $T_{\nu 0}$ (arrow) demarcates the transition of the lightest neutrino from relativistic to nonrelativistic.