collinear factorization. Therefore, the PDFs have an explicit dependence on the transverse momentum of a parton. Such objects are often referred to, as transverse momentum dependent PDFs (TMDs) or Unintegrated PDFs, although the former are typically used outside the small $x$ physics, and posses unambiguous (though in general process dependent) field theoretic definitions. Actually, at small $x$ one usually deals with initial state gluons only, and thus the object of interest in this paper is an Unintegrated Gluon Distribution (UGD). The UGDs have to be convoluted with a perturbative “hard part” according to so-called $k_{T}$ or High Energy Factorization (HEF). We describe this approach in some more detail in Section \[sec:HEF\]. Here, let us just mention that unlike the collinear factorization, the HEF is not a QCD theorem and actually the universality of UGDs is supposed to be violated for jet production in hadron-hadron collisions. Thus, in principle, the standard procedure of fitting the UGDs to the $F_{2}$ HERA data and using it for jets in hadron-hadron collisions is not correct, but there are no quantitative measures of the factorization violation so far. Actually, HEF is surprisingly quite successful with describing LHC data using UGDs from fits to structure functions, 