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

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. 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. 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, 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.