Interaction region of high energy protons

Abstract

New proton–proton collision data from the LHC have considerably extended the energy range over which the structure of the proton–proton interaction region can be studied. In this paper, we combine the unitarity relation with experimental data on elastic scattering in the diffraction cone to show how the shape and the darkness of the inelastic interaction region of colliding protons change with increasing the proton energy. In particular, at LHC energies, small-impact-parameter collisions become fully absorptive, with some implications for inelastic processes as well. The possibility of changing from the black-core scenario at LHC energies to the fully transparent scenario at higher energies is discussed — a phenomenon that implies changing from the black disk to black toroid terminology. As the asymptotic behavior is approached, a different regime may arise. The parameter determining the opacity of central collisions also crucially affects the differential cross section of elastic scattering outside the diffraction cone, where all phenomenological models fail for the LHC energies. It is in this region where the ratio of the real to imaginary part of the elastic scattering amplitude in nonforward scattering becomes a decisive factor, as indeed it should according to the unitarity condition. Our results make it possible for the first time to estimate this ratio outside the diffraction cone by comparing with data for LHC energies, and it turns out to be drastically different from the values measured at forward scattering. Moreover, both real and imaginary parts are found to behave differently in different phenomenological models and in the approach based on the unitarity condition. This problem is still to be resolved. All the conclusions are made solely in the framework of the indubitable unitarity condition using experimental data on elastic proton scattering in the diffraction cone, without resorting to other theoretical methods, such as quantum chromodynamics or phenomenological models.