$\pi^{-}/\pi^{+}$ Ratios of Separated Response Functions in Forward Pion Electroproduction

The first complete separation of the four unpolarized electromagnetic response functions above the dominant resonances has been made for forward, exclusive $\pi^{\pm}$ electroproduction on the nucleon in the $Q^2 = 0.6 - 2.45 $ (GeV/c)$^2$ range. The separated ratio $R_L =\sigma_L^{\pi^-}/\sigma_L^{\pi^+} $ is sensitive to isoscalar contamination to the dominant isovector pion exchange amplitude, which is the basis for the determination of the charged pion form factor, $F_{\pi}(Q^2)$ from electroproduction data. The value of this ratio may also have implications for constraining polarized GPD's with ratios of longitudinal observables. At large $-t$, a separate ratio $R_T = \sigma_T^{\pi^-}/\sigma_T^{\pi^+}\simeq$ 1/4 would suggest a transition between pion and quark knockout mechanisms. Preliminary results on the separate ratios $R_L$ and $R_T$ indicate a dominance of the pion pole diagram at low -t. The results will be compared with a variety of models such as the VGL model.