Dynamics of pp collisions at small impact parameters, universality of cross sections of hadron-hadron collisions and critical phenomena at superhigh energies

Abstract

Studies of the small x dynamics at HERA energies allow one to quantify the range of momentum transfers and energies for which strength of the interaction approaches the maximum strength allowed by the unitarity in the gluon channel (black disk limit-BDL). Implications for the proton-proton interactions at collider energies (Tevatron, LHC) include a dominance of BDL up to transverse momenta of hadrons (jets) in the final state of a few GeV for collisions at small impact parameters, and explanation of proximity of the profile function of pp interaction to one at small impact parameters, leading to the universality of cross sections at superhigh energies. We discuss briey theoretical challenges in the applications of pQCD to small x phenomena, the onset of BDL for hard processes at large energies and universal limiting behavior: F ₂, xG ∝ ln³(x _o/x), σ(γ T) ∝ ln³(s/s _o). We explain that the onset of BDL should be accompanied by generic nonlinear phenomena for the produced states such as kinks, different symmetry of final states, etc and related tunneling transitions. Characteristic features of QCD physics of central collisions at ultrahigh energies correspond to a new regime: (i) interaction is dominated by hard interactions of quarks and gluons leading to disappearance of the soft physics and hadronic degrees of freedom, (ii) change with energy of the space-time evolution of produced quark-gluon states, (iii) change of the avor composition of the produced system, (iv) formation of a color network, etc. Hence, such collisions are likely to result in the production of new forms of the QCD matter. We also comment on the implications of the discussed physics for cosmic ray interactions at ultrahigh energies and on the methods to get information necessary for modeling cosmic rays at energies much higher than LHC using planned experiments at LHC.