Some time ago two of us predicted that at large energies relevant for the LHC the production of double charm should be dominated by the double-parton scattering (DPS) mechanism [1]. Those studies of double $\bar{c}c$ production was extended next to the $k_t$-factorization approach which includes effectively higher-order QCD effects [2, 3]. A relatively good description of the LHCb experimental data [4] was achieved for both the total yield and the dimeson correlation observables. The single-parton scattering (SPS) $gg\to c\bar{c}c\bar{c}$ contribution was discussed carefully in both collinear [3] and $k_t$-factorization [5] approaches. Their contribution to the $c\bar{c}c\bar{c}$ cross section was found to be rather small and was not able to describe details of the LHCb data.

Here we discuss production of $D^0 D^0$ (and $\bar{D^0} \bar{D^0}$) pairs within an alternative approach where g → D fragmentation is included [6]. We consider double-parton scattering (DPS) mechanisms of double $c\bar{c}$ production and subsequent $cc\to D^0 D^0$ hadronization as well as double $g$ and mixed $gc\bar{c}$ production with $gg \to D^0 D^0$ and $gc \to D^0 D^0$ hadronization calculated with the help of the scale-dependent hadronization functions of Kniehl et al. Single-parton scattering (SPS) mechanism of digluon production is also taken into account. We compare our results with several correlation observables in azimuthal angle $\phi_{D^0 D^0}$ between $D^0$ mesons or in dimeson invariant mass $M_{D^0 D^0}$ . The inclusion of new mechanisms with $g \to D^0$ fragmentation leads to larger cross sections, than when including only DPS mechanism with standard scale-independent $cc \to D^0 D^0$ fragmentation functions. Some consequences of the presence of the new mechanisms are discussed. In particular a larger $\sigma_{eff}$ is needed to describe the LHCb data. There is a signature that $\sigma_{eff}$ may depend on transverse momentum of $c$ quarks and/or $\bar{c}$ antiquarks.

[1] M. Luszczak, R. Maciula and A. Szczurek, Phys. Rev. D 85, 094034 (2012).

[2] R. Maciula and A. Szczurek, Phys. Rev. D 87, 074039 (2013).

[3] A. van Hameren, R. Maciula and A. Szczurek, Phys. Rev. D 89, 094019 (2014).

[4] R. Aaij et al. [LHCb Collaboration], J. High Energy Phys. 06, 141 (2012); [J. High Energy Phys. 03, 108 (2014)].

[5] A. van Hameren, R. Maciula and A. Szczurek, Phys. Lett. B 748, 167 (2015).

[6] R. Maciula and V. A. Saleev, A. V. Shipilova and A. Szczurek, arXiv:1601.06981 [hep-ph]