A New Framework for Extracting GPDs from Exclusive Photon Electroproduction

TL;DR

This work extends the SDHEP framework to exclusive real-photon electroproduction off nucleons, providing a two-stage kinematic description that jointly treats BH and DVCS within a single framework. By recasting the process in the SDHEP frame, the authors derive leading and next-to-leading power amplitudes and show that eight polarization observables arise from azimuthal modulations, directly encoding the eight real degrees of freedom of GPD moments. The framework clarifies the frame-dependent structure of Compton form factors and demonstrates the clean separation of soft and hard dynamics, enabling more transparent and potentially model-independent GPD extractions from experimental data. The study also contrasts SDHEP with the conventional Breit frame, arguing that SDHEP offers superior interpretability of azimuthal correlations, while the Breit frame provides a complementary CFF perspective; together, these insights advance the phenomenology of GPDs in nucleons and nuclei.

Abstract

Recently, a new framework for studying generic $2 \to 3$ hard exclusive reactions, referred to as single-diffractive hard exclusive processes (SDHEP), has been introduced to provide a cleaner separation of the underlying physical mechanisms. In this work, we expand this formalism to the case of exclusive real-photon electroproduction off a nucleon, $e(\ell) + N(p) \to e(\ell') + N(p') + γ(q')$, which represents the classical channel for accessing generalized parton distributions (GPDs) in nucleons and nuclei. This extension enables a more systematic and physically transparent formulation of the reaction dynamics, paving the way for improved extractions of GPDs from experimental data as compared to existing approaches.

Figures (13)

• Figure 1: The (a) DVCS and (b) BH subprocesses that make up the photon electroproduction at the LO of QED.
• Figure 2: The two-stage picture of the photon electroproduction process.
• Figure 3: Diagrams of the BH subprocess at the LO of QED.
• Figure 4: Region decomposition of the virtual Compton amplitude. Partons connecting the collinear part $F$ to the hard part $C$ are collinear to the nucleons. The "$\cdots$" stands for regions with more than three partons connecting $C$ and $F$. We have implicitly chosen a physical gauge such that all the gluons displayed are transversely polarized. In a covariant gauge, each diagram can contain an arbitrary number of longitudinally polarized gluons.
• Figure 5: The Breit frame for the photon electroproduction, specifically for the subprocesses (a) DVCS and (b) BH.