Production of two charm quark-antiquark pairs in single-parton scattering within the kt-factorization approach

We present first results for the 2 -> 4 single-parton scattering gg -> c (c) over barc (c) over bar subprocess for the first time fully within the k(t)-factorization approach. In this calculation we have used the Kimber-Martin-Ryskin unintegrated gluon distribution which effectively includes some class of higher-order gluon emissions, and an off-shell matrix element squared calculated using recently developed techniques. The results are compared with our earlier result obtained within the collinear-factorization approach. Only slightly larger cross sections are obtained than in the case of the collinear approach. Inclusion of transverse momenta of gluons entering the hard process leads to a much stronger azimuthal decorrelation between cc and (c) over bar(c) over bar than in the collinear-factorization approach. A comparison to predictions of double parton scattering (DPS) results and the LHCb data strongly suggests that the assumption of two fully independent DPS (gg -> c (c) over bar circle times gg -> c (c) over bar) may be too approximate. (C) 2015 Published by Elsevier B.V.