Abstract
Background: Measurements of forward exclusive meson production at different squared four-momenta of the exchanged virtual photon, , and at different four-momentum transfer, , can be used to probe QCD's transition from meson-nucleon degrees of freedom at long distances to quark-gluon degrees of freedom at short scales. Ratios of separated response functions in and electroproduction are particularly informative. The ratio for transverse photons may allow this transition to be more easily observed, while the ratio for longitudinal photons provides a crucial verification of the assumed pole dominance, needed for reliable extraction of the pion form factor from electroproduction data.
Purpose: We perform the first complete separation of the four unpolarized electromagnetic structure functions in forward, exclusive electroproduction on deuterium above the dominant resonances.
Method: Data were acquired with 2.6–5.2-GeV electron beams and the spectrometers in Jefferson Lab Hall C at central values of 0.6, 1.0, and 1.6 at GeV, and at GeV. There was significant coverage in and , which allowed separation of .
Results: shows a clear signature of the pion pole, with a sharp rise at small . In contrast, is much flatter versus . The longitudinal/transverse ratios evolve with and and at the highest show a slight enhancement for production compared to . The ratio for transverse photons exhibits only a small dependence, following a nearly universal curve with , with a steep transition to a value of about 0.25, consistent with -channel quark knockout. The ratio also drops rapidly with , qualitatively consistent with -channel helicity conservation. The ratio for longitudinal photons indicates a small isoscalar contamination at GeV, consistent with what was observed in our earlier determination of the pion form factor at these kinematics.
Conclusions: The separated cross sections are compared to a variety of theoretical models, which generally describe but have varying success with . Further theoretical input is required to provide a more profound insight into the relevant reaction mechanisms for longitudinal and transverse photons, such as whether the observed transverse ratio is indeed attributable to a transition from pion to quark knockout mechanisms and provide useful information regarding the twist-3 transversity generalized parton distribution, .
9 More- Received 28 August 2014
DOI:https://doi.org/10.1103/PhysRevC.91.015202
©2015 American Physical Society