Deep inelastic scattering at strong coupling from gauge/string duality : the saturation line

For gauge theories which admit a dual string description, we analyze deep inelastic scattering at strong ’t Hooft coupling and high energy, in the vicinity of the unitarity limit. We discuss the onset of unitarity corrections and determine the saturation line which separates weak scattering from strong scattering in the parameter space of rapidity and photon virtuality. We discover that the approach towards unitarity proceeds through two different mechanisms, depending upon the photon virtuality Q2: single Pomeron exchange at relatively low Q2 and, respectively, multiple graviton exchanges at higher Q2. This implies that the total cross–section at high energy and large Q2 is dominated by diffractive processes. This is furthermore suggestive of a partonic description where all the partons have transverse momenta below the saturation momentum and occupation numbers of order one.