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

We present first results for the 2→42→4 single-parton scattering gg→cc¯cc¯ subprocess for the first time fully within the ktkt-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¯c¯ 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→cc¯⊗gg→cc¯) may be too approximate.