Maxwell-Dirac stress-energy tensor in terms of Fierz bilinear currents

TL;DR

This paper derives the stress-energy tensor for the Maxwell-Dirac system using bilinear currents through two independent methods, confirming their equivalence, and explores its reduction under spherical symmetry.

Contribution

It presents a novel derivation of the Maxwell-Dirac stress-energy tensor in bilinear form using Belinfante's and covariant approaches, establishing their agreement.

Findings

The two methods yield identical bilinear stress-energy tensors.

The derived tensor can be reduced under spherical symmetry.

The approach clarifies the role of Fierz identities in the tensor formulation.

Abstract

We analyze the stress-energy tensor for the self-coupled Maxwell-Dirac system in the bilinear current formalism, using two independent approaches. The first method used is that attributed to Belinfante: starting from the spinor form of the action, the well-known canonical stress-energy tensor is augmented, by extending the Noether symmetry current to include contributions from the Lorentz group, to a manifestly symmetric form. This form admits a transcription to bilinear current form. The second method used is the variational derivation based on the covariant coupling to general relativity. The starting point here at the outset is the transcription of the action using, as independent field variables, both the bilinear currents, together with a gauge invariant vector field (a proxy for the electromagnetic vector potential). A central feature of the two constructions is that they both involve the mapping of the Dirac contribution to the stress-energy from the spinor fields to the equivalent set of bilinear tensor currents, through the use of appropriate Fierz identities. Although this mapping is done at quite different stages, nonetheless we find that the two forms of the bilinear stress-energy tensor agree. Finally, as an application, we consider the reduction of the obtained stress-energy tensor in bilinear form, under the assumption of spherical symmetry.