Torus or black disk?

Abstract

It is shown that the interaction region of colliding protons appears almost totally absorbing (black) at impact parameters to 0.4–0.5 fm and LHC energy of 7 TeV. The blackness of the proton interaction region for central interactions is completely defined by the ratio of the diffraction cone slope B in elastic scattering to the total cross section. The corresponding parameter is close to unity at LHC energies. The behavior of this ratio at higher energies will determine whether the interaction region structure will remind a torus or a black disk. Recent phenomenological fittings of experimental data at 7 TeV give no way of distinguishing these cases due to inaccuracies of experiments and uncertainties of the extrapolation to unmeasured regions of transferred momenta.