Measurement of Exclusive $\pi^0$ Electroproduction Structure Functions and their Relationship to Transversity GPDs

TL;DR

This paper reports detailed measurements of exclusive $$ electroproduction structure functions at Jefferson Lab, demonstrating the importance of transversity GPDs for understanding nucleon structure and providing experimental access to these elusive quantities.

Contribution

It provides the first comprehensive measurement of $$ electroproduction structure functions across a wide kinematic range and shows the necessity of including transversity GPDs in theoretical models.

Findings

Transversity GPDs significantly improve agreement between data and theory.

Exclusive $$ electroproduction is sensitive to nucleon helicity flip processes.

Data strongly supports the role of transversity GPDs in nucleon structure analysis.

Abstract

Exclusive $\pi^0$ electroproduction at a beam energy of 5.75 GeV has been measured with the Jefferson Lab CLAS spectrometer. Differential cross sections were measured at more than 1800 kinematic values in $Q^2$, $x_B$, $t$, and $\phi_\pi$, in the $Q^2$ range from 1.0 to 4.6 GeV$^2$,\ $-t$ up to 2 GeV$^2$, and $x_B$ from 0.1 to 0.58. Structure functions $\sigma_T +\epsilon \sigma_L, \sigma_{TT}$ and $\sigma_{LT}$ were extracted as functions of $t$ for each of 17 combinations of $Q^2$ and $x_B$. The data were compared directly with two handbag-based calculations including both longitudinal and transversity GPDs. Inclusion of only longitudinal GPDs very strongly underestimates $\sigma_T +\epsilon \sigma_L$ and fails to account for $\sigma_{TT}$ and $\sigma_{LT}$, while inclusion of transversity GPDs brings the calculations into substantially better agreement with the data. There is very strong sensitivity to the relative contributions of nucleon helicity flip and helicity non-flip processes. The results confirm that exclusive $\pi^0$ electroproduction offers direct experimental access to the transversity GPDs.