Twist-3 GPDs in Deeply-Virtual Compton Scattering

TL;DR

This paper investigates the complexities of twist-3 generalized parton distributions in deeply-virtual Compton scattering, revealing limitations in extracting individual GPDs, the potential discontinuities of twist-3 GPDs, and supporting factorization at this order.

Contribution

It highlights the inability to isolate individual twist-3 GPDs in DVCS and demonstrates the cancellation of discontinuities, advancing understanding of twist-3 effects and factorization.

Findings

Only linear combinations of twist-3 GPDs are accessible in DVCS.

Twist-3 GPDs can be discontinuous in models.

Discontinuities cancel in the DVCS amplitude, supporting factorization.

Abstract

The sub-leading power of the scattering amplitude for deeply-virtual Compton scattering (DVCS) off the nucleon contains leading-twist and twist-3 generalized parton distributions (GPDs). We point out that in DVCS, at twist-3 accuracy, one cannot address any individual twist-3 GPD. This complication appears on top of the deconvolution issues familiar from the twist-2 DVCS amplitude. Accessible are exclusively linear combinations involving both vector and axial-vector twist-3 GPDs. This implies, in particular, that the (kinetic) orbital angular momentum of quarks can hardly be constrained by twist-3 DVCS observables. Moreover, using the quark-target model, we find that twist-3 GPDs can be discontinuous. The discontinuities however cancel in the DVCS amplitude, which further supports the hypothesis of factorization at twist-3 accuracy.