Associated production of Z bosons and b-jets at the LHC in the combined kT + collinear QCD factorization approach

We consider the production of Z bosons associated with beauty quarks at the LHC using a combined k(T) + collinear QCD factorization approach, which interpolates between small x and large x physics. Our consideration is based on the off-shell gluon-gluon fusion subprocess g*g* -> ZQ (Q) over bar at the leading order O(alpha alpha(2)(s)) (where the Z boson further decays into a lepton pair), calculated in the k(T)-factorization approach, and several subleading O(alpha alpha(2)(s)) and O(alpha alpha(3)(s)) subprocesses involving quark-antiquark and quark-gluon interactions, taken into account in conventional (collinear) QCD factorization. The contributions from double parton scattering are discussed as well. The transverse momentum dependent (or unintegrated) gluon densities in a proton are derived from Catani-Ciafaloni-Fiorani-Marchesini (CCFM) evolution equation. We achieve reasonably good agreement with the latest data taken by CMS and ATLAS Collaborations. The comparison of our results with next-to-leading-order pQCD predictions, obtained in the collinear QCD factorization, is presented. We discuss the uncertainties of our calculations and demonstrate the importance of subleading quark-involving contributions in describing the LHC data in the whole kinematic region.