content_image/334261.jpg content_image/334260.jpg content_image/334259.jpg content_image/334257.jpg
need to tune the small ξ asymptotic behavior of Im A ( ξ, t = 0). For this issue let us require the following asymptotic behavior of Im A N − Q$_{0}$ for small ξ : 

<img src='content_image/334261.jpg'>

This choice makes the skewness effect consistent with H1 results. 

In order to parameterize the effect of the contribution of N ( x, t ) − Q$_{0}$ ( x, t ) to the imaginary part of the DVCS amplitude one may try to employ the following class of functions: 

<img src='content_image/334260.jpg'>

The corresponding truly non-forward contribution to the GPD quintessence function N ( x, t ) − Q$_{0}$ ( x, t ) can be recovered form (86) employing the standard Abel transform to- mography procedure and used to compute the corresponding contribution to the real part of the elementary amplitude Re A N − $^{Q$_{0}$}$( ξ, t ). 

Finally, the value of the N − Q$_{0}$ contribution to the D form factor can be computed rule employing the analyticity assumptions with the help of inverse momentum sum rule [12, 13, 32]: 

<img src='content_image/334259.jpg'>

where B is the Euler beta function. 

The analytical properties of D N − $^{Q$_{0}$}$( t ) in the variable t require much attention. The Euler beta function in (87) has poles in t for t = − α + k α ′ with k = 0 , 1 , ... . There is a finite number of “tachion” poles at negative values of t which do not match with the t -channel resonance exchange picture forming the basis for the dual parametrization approach. E.g. for 1 < α < 2 there are two “tachion” poles at t = − α α ′ and t = − α − 1 α ′ . This can be seen as an indication that the simple Regge motivated substitution is inadequate for modelling the t -dependence of the DVCS amplitude. A more sophisticated Ansatz with the non-trivial interplay between the ξ and t dependencies is needed for this issue. 

The interesting possibility is to employ the general form (86) assuming the special form of the t -dependence of C N − $^{Q$_{0}$}$( t ) in order to get rid of “tachion” contributions into the D form factor: 

<img src='content_image/334257.jpg'>

