Physical Review C
Background: Measurements of forward exclusive meson production at different squared four-momenta of the exchanged virtual photon, Q(2), 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 pi(-) and pi(+) 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 pi(+/-) 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 Q(2) values of 0.6, 1.0, and 1.6 GeV2 at W = 1.95 GeV, and Q(2) = 2.45 GeV2 at W = 2.22 GeV. There was significant coverage in phi and is an element of, which allowed separation of sigma(L), T, LT, TT. Results: sigma(L) shows a clear signature of the pion pole, with a sharp rise at small -t. In contrast, sigma(T) is much flatter versus t. The longitudinal/transverse ratios evolve with Q(2) and t and at the highest Q(2) = 2.45 GeV2 show a slight enhancement for pi(-) production compared to pi(+). The pi(-)/pi(+) ratio for transverse photons exhibits only a small Q(2) 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 sigma(TT)/sigma(T) ratio also drops rapidly with Q(2), qualitatively consistent with s-channel helicity conservation. The pi(-)/pi(+) 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 sigma(L) but have varying success with sigma(T). 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, H-T
Huber, G. M., Blok, H. P., Butuceanu, C., Gaskell, D., Horn, T., Mack, D. J., ... & Arrington, J. (2015). Separated response functions in exclusive, forward π±electroproduction on deuterium. Physical Review C, 91(1), 015202.