Separated response functions in exclusive, forward π± electroproduction on deuterium

G. M. Huber et al. (Jefferson Lab F_{π} Collaboration)
Phys. Rev. C 91, 015202 – Published 7 January 2015

Abstract

Background: Measurements of forward exclusive meson production at different squared four-momenta of the exchanged virtual photon, Q2, and at different four-momentum transfer, t, 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 π and π+ 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 L/T/LT/TT in forward, exclusive π± electroproduction on deuterium above the dominant resonances.

Method: Data were acquired with 2.6–5.2-GeV electron beams and the HMS+SOS spectrometers in Jefferson Lab Hall C at central Q2 values of 0.6, 1.0, and 1.6 GeV2 at W=1.95 GeV, and Q2=2.45 GeV2 at W=2.22 GeV. There was significant coverage in ϕ and ε, which allowed separation of σL,T,LT,TT.

Results: σL shows a clear signature of the pion pole, with a sharp rise at small t. In contrast, σT is much flatter versus t. The longitudinal/transverse ratios evolve with Q2 and t and at the highest Q2=2.45 GeV2 show a slight enhancement for π production compared to π+. The π/π+ ratio for transverse photons exhibits only a small Q2 dependence, following a nearly universal curve with t, with a steep transition to a value of about 0.25, consistent with s-channel quark knockout. The σTT/σT ratio also drops rapidly with Q2, qualitatively consistent with s-channel helicity conservation. The π/π+ ratio for longitudinal photons indicates a small isoscalar contamination at W=1.95 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 σL but have varying success with σT. 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, HT.

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  • Received 28 August 2014

DOI:https://doi.org/10.1103/PhysRevC.91.015202

©2015 American Physical Society

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Vol. 91, Iss. 1 — January 2015

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