From Hard to Soft Diffraction and Return

Abstract

The long standing mystery of smallness of diffractive dissociation of hadrons to large effective masses (the Pomeron-proton cross section is only 2 mb) witnesses that the gluonic clouds of valence quarks are so small (r_0=0.3fm) that soft interaction hardly resolves those gluons (diffraction is \propto r_0^4). A color-dipole light-cone (LC) approach is developed which incorporates a strong nonperturbative interaction of the LC gluons. The energy dependent part of the total hadronic cross section is calculated in a parameter-free way employing the nonperturbative LC wave functions of the quark-gluon Fock states. It rises with energy as s^Δ, and we predict Δ=0.17 +/- 0.01, as well as the normalization. However, the energy independent part of the cross section related to inelastic collisions with no gluon radiated (gluons are not resolved) cannot be calculated reliably and we treat it as an adjustable parameter which is fixed fitting just one experimental point for total cross section. Then the energy dependence of the total cross section (the Pomeron part) and the elastic slope are fully predicted, as well as the effective Pomeron trajectory in impact parameter space, in a good agreement with data. These results naturally explain the x-dependence of the diffractive DIS observed at HERA. Although diffraction is expected to be dominated by soft interactions the observed effective Δis about twice as large as one (0.08) known for total cross sections. Diffractive excitations of large effective mass correspond to diffractive gluon radiation and should be associated with our calculated Δ.