Massless fermions in null Fermi coordinates and an application to analog gravity in ${\text{He}}^3$-A

Bogoliubov quasiparticles moving in the background of superfluid ${\text{He}}^3$-A see an apparently curved space-time metric when the background superfluid vacuum is in motion. We study how this curvature couples with the spins of the effectively massless quasiparticles. First, we set up the problem in null Fermi coordinates for radial as well as circular geodesics and then use it in the context of the analog metric seen by the quasiparticles. We obtain an effective magnetic interaction due to curvature coupling, and provide numerical estimates. Some possible implications of these results are then pointed out.