19 tributions (dash-dotted curves in Figure 12) fails in both the p ( γ ∗ , π $^{+}$) n and n ( γ ∗ , π − ) p channels and the behav- ior of the corresponding results (dash-dotted cur ves in Figure 11) is expected for the π − /π + ratio. In Figure 13 (left panel) we compare the resu lts of our calculations with l/t separated p ( γ ∗ , π $^{+}$) n data from [12] with average values of Q 2 = 0 . 35 Ge V 2 and W = 2 . 1 GeV. These data are kinematically close to the real photon point where one does not expect a varia- tion of the pion form factor from its VMD value with Λ 2 γππ = m 2 ρ (770) ≃ 0 . 59 GeV $^{2}$. The curves corr espond to this choice of Λ$_{γππ}$. In the right panel we als o show the l/t separated cross section at Q 2 = 0 . 7 Ge V 2 and W = 2 . 19 GeV. The notations for the curves are the same as in Figure 12. As one can notice, because of diff erent conventions for φ , the signs of the measured interf erence cross section dσ$_{LT}$ are different for the two data s ets. 0 10 20 30 40 50 d σ$^{L}$ /dt [ b/GeV 2 ] 0 10 20 30 0 5 10 15 20 25 d T  /dt [ b/GeV 2 ] 0 3 6 9 12 -8 -4 0 4 8 d TT  /dt [ b/GeV 2 ] -4 -2 0 2 4 0 0.05 0.1 -t [GeV$^{2}$] -5 0 5 10 15 d LT  /dt [ b/GeV 2 ] 0 0.1 0.2 0.3 -t [GeV$^{2}$] -6 -4 -2 0 2 Q$^{2 }$= 0.35 GeV 2 Q$^{2 }$= 0.70 GeV 2 W = 2.10 GeV W = 2.19 GeV DESY DESY The interference pattern between the meson-exc hange and resonance contributions is different in the π + and π − channels. For π − production there are no data f or the longitudinal and transverse cross sections. Howeve r, we conclude that the present model describes rather we ll the available π + and π − data for the unseparated cro ss sec- tions dσ$_{U}$/dt and the interference cross sections d σ$_{TT}$/dt and dσ$_{LT}$/dt . Concerning the π − /π + ratio, we ha ve seen that the resonance contributions are important and that, contrary to the results of Ref. [53], the model bas ed on reggeon exchanges is not compatible with the obs erved ratio $^{1}$. The HERMES data at DESY [7] in exclusive re action p ( e, e ′ π $^{+}$) n extend the kinematic region to higher values of W 2 ≃ 16 GeV 2 toward the DIS region and higher values of − t . At HERMES the kinematic requir ement Q 2 > 1 GeV 2 has been imposed on the scattered e lectron in order to select the hard scattering regime. The r esult- ing range is 1 < Q 2 < 11 GeV 2 and 0 . 02 < x$_{B}$ < 0 . 55 for the Bjorken variable. The measured cross s ections have been integrated over the azimuthal angle φ and a separation of the transverse and longitudinal part s was not feasible. With the 27 . 6 GeV HERA beam energy the ratio of longitudinal to transverse polarization of the virtual photon ε is close to unity. The results for the unseparated differential cros s sec- tions dσ$_{U}$/dt = dσ$_{T}$/dt + εdσ$_{L}$/dt in deeply virtual p ( γ ∗ , π $^{+}$) n reaction at HERMES are shown in Figu re 14. FIG. 13: (Color online) The l/t partial differential cross sec- tions in the reaction p ( γ $^{∗}$, π $^{+}$) n in the kinematics of DESY experiments. The notations for the curves are the same as in Figure 6. The experimental data for values of Q 2 = 0 . 35 GeV 2 are from Ref. [12] and for Q 2 = 0 . 7 GeV 2 are from Ref. [16]. IX. DEEPLY VIRTUAL p ( e, e $^{′}$π $^{+}$) n AT HE R MES 1 Attempting to resolve the latter discrepancy we assumed that in Figures 2 and 3 of Ref. [53] the convention of Ref. [12] is used for φ . This is fine for the data set from [12] in Figures 3 of Ref. [53] but is not correct for the data set from [14] presented in Figures 2 of Ref. [53]. Interestingly, then the π $^{−}$/π + ratios of Ref. [53] are very well reproduced. dashed curves which refer to the contribution of the π - exchange are equal in both channels. The solid curves describe the model results and, as one can see, it is not surprising why the π − /π + ratio of double differential cross sections, which involves all four l/t components, is well reproduced. A model without the resonance con-