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

TL;DR

This paper calculates the production of two charm quark-antiquark pairs via single-parton scattering using the $k_t$-factorization approach, revealing stronger azimuthal decorrelations and challenging the independence assumption of double parton scattering models.

Contribution

First fully $k_t$-factorization calculation of $g g o c ar c c ar c$ process, including higher-order effects and azimuthal correlations, compared to collinear approaches.

Findings

Slightly larger cross sections than collinear approach.

Stronger azimuthal decorrelation between charm pairs.

Double parton scattering assumption may be too simplistic.

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.