Spatial densities of the photon on the light front

TL;DR

This paper calculates the spatial densities of momentum, angular momentum, and pressure for photons on the light front, revealing unique features such as a positive D-term and the distinction between intrinsic and radiation pressures.

Contribution

It provides the first detailed calculation of photon densities on the light front, including polarization effects and the decomposition of form factors into photon and electron contributions.

Findings

Photon D-term is positive, unlike in other particles.

Intrinsic radial pressure of the photon is negative.

Radiation pressure exerted by the photon is positive.

Abstract

The light front densities of momentum, angular momentum, and intrinsic pressure are calculated for the photon, both in the free case and at leading order in quantum electrodynamics. In the latter case, we moreover decompose the form factors into photon and electron contributions. Circularly and linearly polarized photons are both considered, with the latter containing significant azimuthal modulations in both the momentum density and in intrinsic stresses. We find that the D-term of the photon is positive instead of negative, and accordingly the intrinsic radial pressure of the photon is negative. Despite this, the radiation pressure exerted by the photon is positive. We illustrate through explicit calculation how the intrinsic pressure associated with the D-term and the radiation pressure exerted by the photon are different quantities.