The dynamic origins of fermionic D-terms

TL;DR

This paper demonstrates that the D-term for a spin-1/2 fermion arises solely from interactions, vanishing for free fermions, highlighting its dynamical origin and significance for understanding nucleon structure.

Contribution

It reveals that the fermionic D-term is generated by interactions, contrasting with bosonic cases, and provides models illustrating this dynamical origin.

Findings

Fermionic D-term vanishes for free particles.

Interactions induce a non-zero D-term in fermions.

Implications for nucleon structure studies.

Abstract

The D-term is a particle property defined, similarly to the mass and spin, through matrix elements of the energy-momentum tensor. It is currently not known experimentally for any particle, but the D-term of the nucleon can be inferred from studies of hard-exclusive reactions. In this work we show that the D-term of a spin-1/2 fermion is of dynamical origin: it vanishes for a free fermion. This is in pronounced contrast to the bosonic case where already a free spin-0 boson has a non-zero intrinsic D-term as shown in an accompanying work. We illustrate in two simple models how interactions generate the D-term of a fermion with an internal structure, the nucleon. All known matter is composed of elementary fermions. This indicates the importance to study this interesting particle property in more detail, which will provide novel insights especially on the structure of the nucleon.