Paper detail

On the action for Weyssenhoff spin fluid and the Barbero-Immirzi parameter

It was showed by Perez and Rovelli in 2006 that the Holst action in gravity with torsion with massless and minimally coupling Dirac fermions gives rise to the four-fermion coupling term, whose coefficient is a function of the Barbero-Immirzi (BI) parameter. This parameter is present in the Holst action, which is an object of investigation in a non-perturbative formalism of quantum gravity. The key feature is the torsion because its absence implies no effect from Holst term in dynamical equations. In this paper we consider a spin fluid (also called Weyssenhoff fluid), which is a perfect fluid that has intrinsic spin. We study the Host action in gravity with torsion and spin fluid, and we include, for completeness, minimaly coupling massive fermions.We find the equivalent action containing the same four-fermion interaction term previously calculated by Perez and Rovelli without the spin fluid, also the quadratic spin tensor term and finally an interaction term between the axial current and the spin tensor, which can be relevant for new effects and features in Cosmology. In all three cases, the dependence on BI parameter is the same.