Generalised parton distributions at HERA and prospects for COMPASS

TL;DR

This paper develops a model for generalized parton distributions (GPDs) based on a forward ansatz, compares its predictions with HERA DVCS data, and discusses implications for future experiments like COMPASS.

Contribution

It introduces a new GPD model using a forward ansatz and explores its t-dependence, providing predictions for DVCS observables and experimental prospects.

Findings

Model predictions align with H1 and ZEUS DVCS data.

Skewing factor ratios are sensitive to GPD parametrisations.

Differences in t dependence impact predictions for HERMES and COMPASS.

Abstract

We present a model of generalised parton distributions based on a forward ansatz in the DGLAP region. We discuss some aspects of the parametrisations, as the dependence in t, with factorised and non-factorised approaches, where t is the square of the four-momentum exchanged at the hadron vertex. The predictions of this model are then compared with DVCS cross sections from H1 and ZEUS, and a related observable, the skewing factor, defined as the following ratio imaginary amplitudes : $R \equiv {Im {\cal A} (\gamma^*+p \to \gamma +p)\lfrestriction{t=0}} / {Im {\cal A} (\gamma^*+p \to \gamma^* +p)\lfrestriction{t=0}}$. It is an interesting quantity including both the non-forward kinematics and the non-diagonal effects. Finally, we discuss the beam charge asymmetry, which is certainly the most sensitive observable to the different hypothesis needed in the GPDs parametrisations. We show that the approximations done for the t dependence lead to significant differences for the predictions in the HERMES kinematic domain and prospects are given for COMPASS.