CVE for photons: black-hole vs. flat-space derivation

Understand

The chiral vortical effect (CVE) was derived first for non-interacting massless fermions.

• Recently, an alternative derivation of the CVE was suggested which relates it to the radiation from the horizon of a rotating black hole.
• We attempt to generalize the latter derivation to the case of photons and encounter a crucial factor of two difference between the two ways of visualizing the CVE.
• Reservations and possible explanations are briefly discussed.