Quantum electrodynamics of spin 3/2, II

TL;DR

This paper investigates the electromagnetic interactions of spin 3/2 particles with a focus on the transverse spin 1/2 component, ensuring consistency and causality in the theory through a generalized mass term and detailed quantization analysis.

Contribution

It introduces a more general mass term allowing the transverse spin 1/2 component to propagate, and demonstrates the theory's consistency, causality, and unitarity with detailed pole and residue analysis.

Findings

Interaction is consistent without superluminal propagation for certain mass ranges.

The propagator's poles and residues are unconventional but lead to a unitary S-matrix.

The quantized theory is causal with a positive definite anti-commutator.

Abstract

Electromagnetic interactions of the spin 3/2 particle are investigated while allowing the propagation of the transverse spin 1/2 component present in the reducible Rarita-Schwinger vector-spinor. This is done by allowing a more general form for the mass term, while leaving the kinetic terms untouched. We find that the interaction is consistent and does not lead to superluminal propagation for a range of the mass of the spin 1/2 particle, except for the special point where the spin 1/2 particle is infinitely massive. We then quantize the theory using the appropriate Grassmann-variable path integral and study the poles of the propagator. The unfamiliar feature of the theory is that the charge matrix is not positive definite; it is positive definite on the space of spin 3/2 solutions, and negative definite on the space of spin 1/2 solutions. Classically, for spin 1/2 modes the energy is of the opposite sign to frequency, i.e. the positive frequency modes have negative energy. It is found that the retarded part appropriately propagates the positive energy solutions forward in time, while the advanced part propagates the negative energy solutions back in time so long as the free parameter is chosen such that the positive frequency modes have the same parity. The propagator contains poles of the spin 1/2 modes above the real axis for the positive and below the real axis for the negative frequency modes, while the residues at those poles are also of the sign opposite to the usual; altogether this leads to a unitary S-matrix, the forward amplitude <p|p> being positive for all modes. The canonically quantized field is causal, and the equal-time anti-commutator has positive definite form. We choose the physical vacuum as the highest-weight state with respect to the spin 1/2 operators in order to have positive energy, and to correctly reproduce both the unusual residues and locations of the poles.