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n al εσ$_{L}$ and the transverse σ$_{T}$ components, respectively. 

In the region of small Bjorken x$_{B}$ , see left panel in Fig- ure 19, the integrated longitudinal component dominates over the transverse cross section. With increasing x$_{B}$ the strength of the transverse component is increasing and for values of x$_{B}$ in the third bin the transverse cross sec- tion σ$_{T}$ becomes the dominant part of the exclusive cross section. An increase of the relative contribution of σ$_{T}$ as a function of x$_{B}$ can be clearly seen in the right panel of Figure 19 where 0 . 26 < x$_{B}$ < 0 . 55. There the first Q 2 bin corresponds to the average value of x$_{B}$ = 0 . 29 and the last Q 2 bin to the average value of x$_{B}$ = 0 . 44 [57]. 

In this work we have restricted ourselves to exclusive ( e, e ′ π ± ) reactions with an unpolarized target. With a polarized beam ( ⃗ e, e ′ π ± ) and with an unpolarized target there is an additional component σ$_{LT}$ ′ , Eq. (3), which is proportional to the imaginary part of an interference between the l/t photons and therefore sensitive to the relative phases of amplitudes. 

In general, a nonzero σ$_{LT}$ ′ or the corresponding beam SSA A$_{LU}$ ( φ ), Eq. (7), demands interference between sin- gle helicity flip and nonflip or double helicity flip ampli- tudes. In Regge models the asymmetry may result from Regge cut corrections to single reggeon exchange [58]. This way the amplitudes in the product acquire different phases and therefore relative imaginary parts. A nonzero beam SSA can be also generated by the interference pat- 

## XI. BEAM SPIN ASYMMETRY 

tern of amplitudes where particles with opposite parities are exchanged. 

In the following we discuss briefly the generic features of the beam SSA in the present model. The compara- tive analysis of the SSA at JLAB and HERMES will be presented in the forthcoming publication. 

In the left panel of Figure (20) we plot the CLAS data [6] for the azimuthal moment A sin( φ ) LU associated with the beam SSA, Eq. (8), in the reaction p ( ⃗ e, e ′ π $^{+}$) n . These data have been collected in hard scattering kinematics E$_{e}$ = 5 . 77 GeV, W > 2 GeV and Q 2 > 1 . 5 GeV $^{2}$. The experiment shows a sizable and positive beam SSA. In the left and right panels of Figure (20) we present the results for the azimuthal moments A sin( φ ) LU in the re- actions p ( ⃗ e, e ′ π $^{+}$) n and n ( ⃗ e, e ′ π − ) p , respectively. The ( Q $^{2}$, W ) binning of the experimental data point is not available. In the following the calculations are done for the lowest ( Q $^{2}$, W ) bin corresponding to the values of Q 2 = 1 . 5 GeV 2 and W 2 = 4 GeV $^{2}$. 

At first, we consider A sin( φ ) LU generated by the ex- change of Regge trajectories. In Figure (20) the dashed curves describe the model results without the r/p -effects and neglecting the exchange of the axial-vector a$_{1}$ (1260) Regge trajectory. This model results in a zero A sin( φ ) LU and therefore a zero beam SSA. The addition of the un- natural parity a$_{1}$ (1260)-exchange generates by the inter- ference with the natural parity ρ (770) exchange a siz- able A sin( φ ) LU in both channels. This result corresponds to the dash-dotted curves in Figure (20). In the rest of observables discussed above the effect of the axial- 

