Physical Review C
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 GeV2at 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 Q2dependence, 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.
Original Publication Citation
Huber, G. M., Blok, H. P., Butuceanu, C., Gaskell, D., Horn, T., Mack, D. J., . . . Jefferson Lab, F. C. (2015). Separated response functions in exclusive, forward π± electroproduction on deuterium Physical Review C, 91(1), 015202. doi:10.1103/PhysRevC.91.015202
Huber, G. M.; Blok, H. P.; Klein, A.; Qin, L. M.; Vansyoc, K.; and Jefferson Lab, Fπ Collaboration, "Separated Response Functions in Exclusive, Forward π± Electroproduction on Deuterium" (2015). Physics Faculty Publications. 151.