Exclusive $\rho^0$ electroproduction on the proton at CLAS

TL;DR

This paper presents extensive data on exclusive $ ho^0$ electroproduction on the proton at CLAS, comparing experimental results with theoretical models based on Regge theory and GPDs, revealing discrepancies and raising questions about the applicability of GPDs in this kinematic region.

Contribution

It provides the largest dataset for this reaction in the valence region and critically compares experimental results with two theoretical approaches, highlighting limitations of current GPD models.

Findings

Regge theory describes data at about 30% accuracy

GPD-based models underestimate the data significantly

Discrepancies suggest possible limitations of GPD formalism in this region

Abstract

The $e p\to e^\prime p \rho^0$ reaction has been measured, using the 5.754 GeV electron beam of Jefferson Lab and the CLAS detector. This represents the largest ever set of data for this reaction in the valence region. Integrated and differential cross sections are presented. The $W$, $Q^2$ and $t$ dependences of the cross section are compared to theoretical calculations based on $t$-channel meson-exchange Regge theory on the one hand and on quark handbag diagrams related to Generalized Parton Distributions (GPDs) on the other hand. The Regge approach can describe at the $\approx$ 30% level most of the features of the present data while the two GPD calculations that are presented in this article which succesfully reproduce the high energy data strongly underestimate the present data. The question is then raised whether this discrepancy originates from an incomplete or inexact way of modelling the GPDs or the associated hard scattering amplitude or whether the GPD formalism is simply inapplicable in this region due to higher-twists contributions, incalculable at present.