Subdominant terms in the production of $c \bar c$ pairs in proton-proton collisions

TL;DR

This paper investigates subleading mechanisms in charm quark pair production in proton-proton collisions at high energies, highlighting the limitations of current gluon distribution knowledge and comparing various production processes including diffractive and exclusive mechanisms.

Contribution

It introduces a comprehensive analysis of subleading production mechanisms for $c ar c$ pairs, incorporating photon-induced processes and diffractive contributions using MRST-QED distributions.

Findings

Subleading mechanisms significantly affect $c ar c$ production predictions.

Photon-induced processes contribute notably at forward rapidities.

Diffractive and exclusive processes have distinct kinematic signatures.

Abstract

At high-energies the gluon-gluon fusion is the dominant mechanism of $c \bar c$ production. This process was calculated in the NLO collinear as well as in the k$_t$-factorization approaches in the past. We show that the present knowledge of gluon distributions does not allow to make a precise predictions for $c \bar c$ production at LHC, in particular at forward rapidities. In this paper we study production of $c \bar c$ pairs including several subleading mechanisms. This includes: $gg \to Q \bar Q$, $\gamma g \to Q \bar Q$, $g \gamma \to Q \bar Q$, $\gamma \gamma \to Q \bar Q$. In this context we use MRST-QED parton distributions which include photon as a parton in the proton as well as elastic photon distributions calculated in the equivalent photon approximation. We present distributions in the $c$ quark ($\bar c$ antiquark) rapidity and transverse momenta and compare them to the dominant gluon-gluon fusion contribution. We discuss also inclusive single and central diffractive processes using diffractive parton distribution found from the analysis of HERA diffractive data. As in the previous case we present distribution in $c$ ($\bar c$) rapidity and transverse momentum. Finally we present results for exclusive central diffractive mechanism discussed recently in the literature. We show corresponding differential distributions and compare them with corresponding distributions for single and central diffractive components.