Relativistically invariant method for constructing the one-and the two-parton density matrix

By representing the hadron as a system consisting of one or two partons and a multiparton core, which in turn is treated as a parton of variable mass, a relativistically invariant density matrix for this system is constructed by performing integration with respect to this mass. The proposed approach makes it possible to establish simple relations between the density-matrix elements and to validate or interpret more clearly assumptions adopted by various authors in parton models.