photons polarized parallel to the reaction plane, one can directly access the difference between recoil and the po- larized target asymmetries which is proportional to the exchange of the a$_{1}$ (1260)-trajectory [44]. However, being suppressed by the Regge factor ∼ $_{e}$− α ′ a 1 ln( α ′ a $_{1}$s ) m 2 a 1 at t = 0, its contribution to the forward unpolarized cross section turns out to be small. With our choice of the b$_{1}$NN tensor coupling the contribution of b$_{1}$ (1235) ex- change is even smaller. On the other hand, as we shall see, it is absolutely essential to consider the exchange of a$_{1}$ (1260) Regge trajectory in the polarization ( ⃗ e, e ′ π ) observables like the beam spin azimuthal asymmetry con- sidered in the following. Other aspects related to a pos- sible role of a$_{1}$ (1260) in ( e, e ′ π ) are discussed in [45]. occur for even or odd positive integer value of the tra- jectory α ( t ) = J (the spin). The leading mesons con- tributing to ( e, e ′ π ± ) are the natural P = ( − 1) J parity vector ρ (770) and the unnatural P = ( − 1) J +1 parity axial-vector mesons a$_{1}$ (1260) and b$_{1}$ (1235). The Regge trajectories α ( t ) considered here are shown in Figure 3 The absolute contribution of the reggeized ρ -exchange amplitude to ( e, e ′ π ± ) turns out to be small, but by its interference with the s - and u -channel terms consid- ered above, it is responsible for the π − /π + asymmetry in photoproduction and gives a sizable contribution to the π − /π + ratio in electroproduction. In the axial-vector sector, the experimental isolation of the amplitudes with axial-vector quantum numbers would be of great interest. For instance, using a proton target polarized perpendicular to the reaction plane, and 7 The currents J µ ρ describing the exchange of the natural parity ρ (770)-meson Regge trajectory are given by where G$_{ρNN}$ = 3 . 4 and κ$_{ρ}$ = 6 . 1 are the standard vector and anomalous tensor coupling constants, respectively. The ρ -trajectory adopted here reads α$_{ρ}$ ( t ) = 0 . 53+ α ′ $_{ρ}$t with a slope α ′ ρ = 0 . 85 GeV − $^{2}$. The Γ function in (33) contains the pole propagator ∼ 1 / sin( πα$_{ρ}$ ( t )) but no zeroes and the amplitude zeroes only occur through the factor 1 − e − iπα$_{ρ}$ ( t $^{)}$. The ργπ coupling constants G$_{ργπ}$ can be deduced from the radiative γπ decay widths of ρ The measured width [46]: Γ$_{ρ}$ $^{±}$→ γπ ± = (68 ± 7) keV , where the central value corresponds to G$_{ργπ}$ = 0 . 728 GeV − $^{1}$. For the transition form factor F$_{ργπ}$ ( Q $^{2}$) we use a VMD model F$_{ργπ}$ ( Q $^{2}$) = (1 + Q $^{2}$/ Λ 2 $_{ργπ}$) − 1 with Λ$_{ργπ}$ = m$_{ω}$$_{(782)}$ . A. Vector-isovector I $^{G}$( J $^{PC}$) = 1 $^{+}$(1 $^{−−}$) exchange currents B. Axial-vector I $^{G}$( J $^{PC}$) = 1 $^{−}$(1 $^{++}$) exchange currents   − iJ µ $_{ρ}$( γ ∗ p → π $^{+}$n ) − iJ µ $_{ρ}$( γ ∗ n → π − p )   = − i √$_{2}$$_{G}$$_{ργπ}$ G$_{ρNN}$ F$_{ργπ}$ ( Q $^{2}$) ε $^{µναβ}$q$_{ν}$ k$_{α}$ ¯ u$_{s}$ ′ ( $_{p}$′$_{)}$ [ (1 + κ$_{ρ}$ ) γ$_{β}$ − κ$_{ρ}$ 2 M$_{p}$ ( p + $_{p}$′$_{)}$$_{β}$ ] u$_{s}$ ( p ) × [ 1 − e − iπα$_{ρ}$ ( t ) 2 ] ( − α ′ ρ ) Γ[1 − α$_{ρ}$ ( t )] e ln( α ′ $_{ρ}$s )( α$_{ρ}$ ( t $^{)}$−1) (33) Γ( ρ ± → γπ ± ) = α$_{e}$ 24 G 2 ργπ m 3 ρ ( m 2 ρ − m 2 π ) 3 . (34) A$_{µ}$ = ( a 0 1 √$_{2}$$_{a}$ + 1 √$_{2}$$_{a}$$_{−}$ 1 − a 0 1 ) µ . (35) L a$_{1}$NN = G$_{a}$$_{1}$$_{NN}$ ¯ ψγ $^{µ}$γ$_{5}$ A$_{µ}$ψ, (36) The axial-vector a$_{1}$ (1260) meson with I $^{G}$( J $^{P C}$) = 1 − (1 $^{++}$) has a large width into the a$_{1}$ (1260) ± → γπ ± channel [46]. A conversion of a$_{1}$ into γπ is described by the a$_{1}$γπ vertex L a$_{1}$γπ = − ie 4 G$_{a}$$_{1}$$_{γπ}$F $^{µν}$〈 Q [ A$_{µν}$, ϕ ] 〉 where F µν denotes the field tensor of photons, A$_{µν}$ stands for the field tensor of the axial-vector meson with A$_{µν}$ = ∂$_{µ}$A$_{ν}$ − ∂$_{ν}$A$_{µ}$ and ϕ is a standard SU (2) pion matrix, Q = diag(2 / 3 , − 1 / 3) is a quark charge matrix, 〈 .. 〉 and [ .. ] denote a trace and a commutator of fields. The hadronic a$_{1}$NN interaction is described by where ψ = ( p, n ) $^{T}$. Because of G -parity conservation in the vertex there is no tensor coupling of a$_{1}$ to nucleons.