Distinguishing Dirac vs. Majorana Neutrinos: a Cosmological Probe

Cosmic background neutrinos ($C_{\nu}B)$ helicity composition is different for Dirac or Majorana neutrinos making detectors based on $C_{\nu}B$ capture sensitive to the nature of neutrinos. We calculate, for the first time, the helicity changes of neutrinos crossing dark matter fields, to quantitatively calculate this effect on the capture rate. We show that a fraction of neutrinos change their helicity, regardless of them being deflected by a void or a dark matter halo. The average signal from the 100 most massive voids or halos in a Gpc$^3$ gives a prediction that if neutrinos are Dirac, the density of the $C_{\nu} B$ background measured on Earth should be 48 cm${^{-3}}$ for left-helical neutrinos, a decrease of 15% (53.6 cm${^{-3}}$; 5%) for a halo (void) with respect to the standard calculation without including gravitational effects due to large scale structures. In terms of the total capture rate in a 100 g tritium detector, this translates in $4.9^{+1.1}_{-0.8}$ neutrinos per year for the Dirac case, as a function of the unknown neutrino mass scale, or 8.1 per year if neutrinos are Majorana. Thus although smaller than the factor two for the non-relativistic case, it is still large enough to be detected and it highlights the power of future $C_{\nu} B$ detectors, as an alternative to neutrinoless double beta decay experiments, to discover the neutrino nature.