Presumably this should be a tiny cross-section (or not?), but presumably it should be at least qualitatively similar to the 511 keV line that clearly distinguishes electron-positron annihilation, although by the upper mass bounds on the different types of neutrinos, such line should be presumably be somewhere in the microwave region or below. Even if the rate is tiny, it could be detectable cosmologically, if we knew in what region of spectra to look
On the other hand, considering that the inverse process (photon to neutrino-antineutrino pair $\gamma \gamma \rightarrow \nu \bar{\nu} $) could only be experimentally tested with left-handed $\nu$, right-handed $\bar{\nu}$ pairs, we should expect to see at least half the rate since theoretically the pair production channel toward right-handed-neutrino-left-handed-antineutrino pairs should not be suppressed in this case, correct? and at least from our galactic frame of references, those chiralities should behave as sterile neutrinos
