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, R_A/p=f_i/A^D3/f_i/A/f_i/p^D3/f_i/p, for coherent and summed (coherent plus quasi-elastic) nuclear deep-inelastic scattering. We find that R_A/p ≈ 0.5 − 1 for quarks as well as for the ratio of the diffractive and total cross sections dσdiff/dMX2/σtot_eA/dσdiff/dMX2/σtot_ep and R_A/p ≈ 0.5 − 1.3 for gluons in a broad range of x, including the kinematics of the Electron-Ion Collider, which reaffirms the difference from the nuclear enhancement of R_A/p predicted in the gluon saturation framework. We demonstrate that the magnitude of R_A/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 R_A/p in the color dipole model and its suppression in LTA. We argue that the black disk limit corresponds to R_A/p = 1 and R_A/p^coh = 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 Q_sA²b/Q_sp²b≈1 at small impact parameters b due to the strong leading twist nuclear shadowing and diluteness of the nuclear density.