Suppression of diffraction in deep-inelastic scattering on nuclei and dynamical mechanism of leading twist nuclear shadowing

Using the leading twist approach (LTA) to nuclear shadowing, we calculate the ratios of diffractive and usual parton distributions for a heavy nucleus (Pb) and the proton, RA/p=(fi/AD(3)/fi/A)/(fi/pD(3)/fi/p), for coherent and summed (coherent plus quasi-elastic) nuclear deep-inelastic scattering. We find that RA/p ≈ 0.5-1 for quarks and RA/p ≈ 0.5-1.3 for gluons in a broad range of x, which reaffirms the difference from the nuclear enhancement of RA/p predicted in the gluon saturation framework. We demonstrate that the magnitude of RA/p is controlled by the cross section of the interaction of hadronic fluctuations of the virtual photon with target nucleons, which explains an enhancement of RA/p in the color dipole model and its suppression in LTA. We argue that the black disk limit of LTA corresponds to RA/p=1 and RᶜᵒʰA/p=0.86 for the summed and coherent scattering, respectively. Relying on an intuitive definition of the saturation scale, we show that the ratio of the saturation scales of a heavy nucleus and proton QsA²(b)/Qₛₚ²(b) ≈ 1 at small impact parameters b due to the strong leading twist nuclear shadowing and diluteness of the nuclear density.