  We investigate the effective field theory (EFT) which gives the approximate
description of the scattering of the two hard small dipoles in the small $x$
processes in QCD near the black disc limit(BDL). We argue that the perturbative
QCD approaches predict the existence of tachyon and visualize it in the
approximation where $\alpha'_P=0$ . We demonstrate that the high energy
behavior of the cross-section depends strongly on the diffusion law in the
impact parameter plane. On the other hand, almost threshold behavior of the
cross section of the hard processes and multiplicities, i.e. fast increase of
cross sections (color inflation), melting of ladders into color network and
softening of the longitudinal distributions of hadrons are qualitatively
insensitive to the value of diffusion in the impact parameter space. We
evaluate $\alpha'_P$ near the black disk limit and find significant $\alpha'_P$
as the consequence of the probability conservation.
