Revisiting the production of $J/\psi$ pairs at the LHC

TL;DR

This paper analyzes prompt double $J/\psi$ production at the LHC using a $k_T$-factorization approach, highlighting the significance of color octet and singlet mechanisms and double parton scattering in different kinematic regions.

Contribution

It provides a comprehensive evaluation of color octet contributions with initial state gluon radiation effects and compares different production mechanisms against experimental data.

Findings

Color octet contributions are significant in certain kinematic regions.

Color singlet and double parton scattering mechanisms explain forward rapidity data.

The effective cross section $\sigma_{\rm eff}$ is estimated as 17.5 mb.

Abstract

We consider the prompt double $J/\psi$ production in $pp$ collisions at the LHC in the framework of $k_T$-factorization QCD approach. Using the fragmentation mechanism, we evaluate the color octet contributions to the production cross sections taking into account the combinatorial effects of multiple gluon radiation in the initial state driven by the Ciafaloni-Catani-Fiorani-Marchesini evolution equation. We demonstrate the importance of these contributions in a certain kinematical region covered by the CMS and ATLAS measurements. On the other hand, the experimental data taken by the LHCb Collaboration at forward rapidities and moderate transverse momenta can be described well by ${\cal O}(\alpha_s^4)$ color singlet terms and contributions from the double parton scattering mechanism. The extracted value of the effective cross section $\sigma_{\rm eff} = 17.5 \pm 4.1$~mb is compatible with many other estimations based on different final states.