Speaker
Description
We study the small-$x$ asymptotics of unpolarized generalized parton distributions (GPDs). Unlike the previous works in the literature, we consider the case of non-zero skewness. We show that the unpolarized GPDs at small $x$ are related to the eikonal dipole amplitude $N$, whose small-$x$ evolution is given by the BK/JIMWLK evolution equations. We show that the effect of non-zero skewness $\xi \neq 0$ is to modify the value of the evolution parameter (rapidity) in the argument for the dipole amplitude from $Y = \ln (1/x)$ to $Y = \ln \min \left\{ 1/x , 1/|\xi| \right\}$.
Further, we address the question of calculating the real part of the scattering amplitude at high energies, corresponding to the imaginary part of the dipole amplitude $N$. In phenomenology, this real part is often accounted for by a multiplicative $R$-factor, often used in elastic vector meson production calculations. We study the origin of the $R$-factor in the shock wave picture and find that the real part of the scattering amplitude originates from multiple $t$-channel gluon exchanges in the initial conditions for the small-$x$ evolution. We show that such exchanges are the origin of the signature factor in the scattering amplitudes in the shock wave approach to high-energy scattering.
