Revisiting canonical gravity with fermions

TL;DR

This paper revisits the classical formulation of gravity with fermions, emphasizing background-free quantization, and explores the implications of parity and invariance properties in dynamical gravity.

Contribution

It provides a detailed analysis of the Hilbert-Palatini formulation with the Nieh-Yan term, clarifying the role of the Barbero-Immirzi parameter and parity in quantum gravity with fermions.

Findings

The Nieh-Yan term maintains Barbero-Immirzi parameter independence.

Distinct notions of parity in dynamical gravity are identified and analyzed.

Invariance properties differ for orientation reversing diffeomorphisms and improper Lorentz rotations.

Abstract

Fermions constitute an important component of matter and their quantization in presence of dynamical gravity is essential for any theory of quantum gravity. We revisit the classical formulation adapted for a background free quantization. The analysis is carried out with the Hilbert-Palatini form for gravity together with the Nieh-Yan topological term which keeps the nature of Barbero-Immirzi parameter independent of inclusion of arbitrary matter with arbitrary couplings. With dynamical gravity, a priori, there are two distinct notions of `parity' - orientation reversing diffeomorphisms and improper Lorentz rotations. The invariance properties of the action and the canonical framework are different with respect to these and gravitational origin of parity violation seems ambiguous.