Production of two charm quark-antiquark pairs in single-parton scattering within the $k_t$-factorization approach
Andreas van Hameren, Rafal Maciula, Antoni Szczurek
TL;DR
This paper extends SPS calculations for pp -> ccbar ccbar X into the kt-factorization framework, using the KMR UGDF to effectively incorporate higher-order gluon emissions. It provides the first fully kt-factorized 2->4 calculation with two off-shell initial gluons and compares results to collinear-factorization predictions. The results show cross sections are only modestly larger than the collinear case, but transverse gluon momenta lead to strong azimuthal decorrelations between the two charm-quark pairs. Comparisons with DPS expectations and LHCb data indicate that treating the two DPS subprocesses as fully independent may be overly simplistic, underscoring the SPS contribution as a crucial irreducible background.
Abstract
We present first results for the $2 \to 4$ single-parton scattering $g g \to c \bar c c \bar c$ 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 $c c$ 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 ($g g \to c \bar c \otimes g g \to c \bar c$) may be too approximate.