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We derive expressions for the cross section of the multiparton interactionsbased on the analysis of the relevant Feynman diagrams. We express the crosssections through the double (triple, ...) generalized parton distributions(GPDs). In the mean field approximation for the double GPDs the answer isexpressed through the integral over two gluon form factor which was measured inthe exclusive DIS vector meson production.We explain under what conditions thederived expressions correspond to an intuitive picture of hard interactions inthe impact parameter representation. The mean field approximation in whichcorrelations of the partons are neglected fail to explain the data, while pQCDinduced correlation enhance large $p_\perp$ and $ 0.001 < x < 0.1$ typicallyenhance the cross section by a factor of 1.5 -- 2 explaining the current data.We argue that in the small x kinematics ($10^{-4} \le x \le 10^{-3}$) whereeffects of perturbative correlations diminish, the nonperturbative mechanismkicks in and generates positive correlations comparable in magnitude with theperturbative ones. We explain how our technique can be used for calculations ofMPI in the proton - nucleus scattering. The interplay of hard interactions andunderlying event is discussed, as well as different geometric pictures for eachof MPI mechanisms-pQCD, nonperturbative correlations and mean field.Predictions for value of \effs for various processes and a wide range ofkinematics are given. We show that together different MPI mechanisms give gooddescription of experimental data, both at Tvatron, and LHC, including thecentral kinematics studied by ATLAS and CMS detectors, and forward (heavyflavors) kinematics studied by LHCb.

Multiparton pp and pA collisions -- from geometry to parton-- parton correlations