Dirac and Majorana neutrino signatures of primordial black holes
  We study Primordial Black Holes (PBHs) as sources of massive neutrinos via
Hawking radiation. Under the hypothesis that black holes emit neutrino mass
eigenstates, we describe quantitatively how the PBH evolution and lifetime is
affected by the mass and fermionic -- Dirac or Majorana -- nature of neutrinos.
In the case of Dirac neutrinos, PBHs radiate right-handed and left-handed
neutrinos in equal amounts, thus possibly increasing the effective number of
neutrino species, $N_{\rm eff}$. Assuming an initially monochromatic PBH mass
spectrum, with the initial mass $M_i$ related to the particle horizon mass, and
considering the current constraint on $N_{\rm eff}$, we derive a bound on the
initial PBH fraction $\beta^\prime$ in the interval $4.3\times 10^7\ {\rm
g}\lesssim M_i \lesssim 10^9$ g. Future measurements of $N_{\rm eff}$ may be
able to constraint the initial fraction for black hole masses as low as 1 g. If
an excess in $N_{\rm eff}$ is found, PBHs with Dirac neutrinos could provide a
minimal explanation of it. For example, for $10^7\ {\rm g} \lesssim M_i\lesssim
10^9$ g and $\beta^\prime \gtrsim 10^{-13}$, an excess radiation at the level
of $0.2\lesssim \Delta N_{\rm eff}\lesssim 0.37$ is produced, which can
alleviate the tension of the Hubble parameter measurements. Finally, we obtain
the diffuse flux of right-helical neutrinos from PBHs at the Earth, and show
that their detection in a PTOLEMY-like detector (using neutrino capture on
tritium) would be difficult.
