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

Abstract

We present first results for the $2\to 4$ single-parton scattering $gg\to c\bar{c}c\bar{c}$ 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 $\bar{c}\bar{c}$ 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 ($gg\to c\bar{c}\otimes gg\to c\bar{c}$) may be too approximate.

Comments: 14 pages, 12 figures