Massless relativistic wave equations and quantum field theory

TL;DR

This paper constructs massless quantum fields with arbitrary helicity satisfying Wightman axioms, highlighting mathematical differences from massive models and analyzing covariant and canonical representations of the Poincare' group.

Contribution

It provides a direct construction of massless quantum fields with arbitrary helicity using the massless free net and detailed analysis of Poincare' group representations, emphasizing differences from massive models.

Findings

Massless models with nontrivial helicity have non-coinciding covariant and canonical fibre degrees of freedom.

Massless relativistic wave equations act as constraint equations reducing fibre degrees of freedom.

The construction uses intertwiner spaces between E(2) and its conjugate to define the one-particle Hilbert space.

Abstract

We give a simple and direct construction of a massless quantum field with arbitrary discrete helicity that satisfies Wightman axioms and the corresponding relativistic wave equation in the distributional sense. We underline the mathematical differences to massive models. The construction is based on the notion of massless free net (cf. Section 3) and the detailed analysis of covariant and massless canonical (Wigner) representations of the Poincare' group. A characteristic feature of massless models with nontrivial helicity is the fact that the fibre degrees of freedom of the covariant and canonical representations do not coincide. We use massless relativistic wave equations as constraint equations reducing the fibre degrees of freedom of the covariant representation. They are characterized by invariant (and in contrast with the massive case non reducing) one-dimensional projections. The definition of one-particle Hilbert space structure that specifies the quantum field uses distinguished elements of the intertwiner space between E(2) (the two-fold cover of the 2-dimensional Euclidean group) and its conjugate. We conclude with a brief comparison between the free nets constructed in Section 3 and a recent alternative construction that uses the notion of modular localization.