Measurement of Exclusive<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mi>π</mml:mi><mml:mn>0</mml:mn></mml:msup></mml:math>Electroproduction Structure Functions and their Relationship to Transverse Generalized Parton Distributions

Abstract

Exclusive ${\ensuremath{\pi}}^{0}$ 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 ${\ensuremath{\phi}}_{\ensuremath{\pi}}$, in the ${Q}^{2}$ range from 1.0 to $4.6\text{ }\text{ }{\mathrm{GeV}}^{2}$, $\ensuremath{-}t$ up to $2\text{ }\text{ }{\mathrm{GeV}}^{2}$, and ${x}_{B}$ from 0.1 to 0.58. Structure functions ${\ensuremath{\sigma}}_{T}+ϵ{\ensuremath{\sigma}}_{L}$, ${\ensuremath{\sigma}}_{TT}$, and ${\ensuremath{\sigma}}_{LT}$ 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 generalized parton distributions (GPDs). Inclusion of only longitudinal GPDs very strongly underestimates ${\ensuremath{\sigma}}_{T}+ϵ{\ensuremath{\sigma}}_{L}$ and fails to account for ${\ensuremath{\sigma}}_{TT}$ and ${\ensuremath{\sigma}}_{LT}$, 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 nonflip processes. The results confirm that exclusive ${\ensuremath{\pi}}^{0}$ electroproduction offers direct experimental access to the transversity GPDs.