Intrinsic charm in the nucleon and charm production at large rapidities in collinear, hybrid and $k_T$-factorization approaches

TL;DR

This paper investigates the impact of intrinsic charm in the nucleon on forward charm production in proton-proton collisions across various energies, comparing different theoretical approaches and highlighting implications for high-energy neutrino experiments.

Contribution

It provides a comprehensive analysis of intrinsic charm effects using collinear, hybrid, and $k_T$-factorization approaches, including new calculations of rapidity and transverse momentum distributions.

Findings

Intrinsic charm dominates at large rapidities.

$k_T$-factorization yields larger cross sections than LO collinear.

Implications for neutrino production in IceCube and LHC experiments.

Abstract

We discuss the role of intrinsic charm (IC) in the nucleon for forward production of $c$-quark (or $\bar c$-antiquark) in proton-proton collisions for low and high energies. The calculations are performed in collinear-factorization approach with on-shell partons, $k_T$-factorization approach with off-shell partons as well as in a hybrid approach using collinear charm distributions and unintegrated (transverse momentum dependent) gluon distributions. For the collinear-factorization approach we use matrix elements for both massless and massive charm quarks/antiquarks. The distributions in rapidity and transverse momentum of charm quark/antiquark are shown for a few different models of IC. Forward charm production is dominated by $gc$-fusion processes. The IC contribution dominates over the standard pQCD (extrinsic) $gg$-fusion mechanism of $c\bar c$-pair production at large rapidities or Feynman-$x_F$. We perform similar calculations within leading-order and next-to-leading order $k_T$-factorization approach. The $k_T$-factorization approach leads to much larger cross sections than the LO collinear approach. At high energies and large rapidities of $c$-quark or $\bar c$-antiquark one tests gluon distributions at extremely small $x$. The IC contribution has important consequences for high-energy neutrino production in the Ice-Cube experiment and can be, to some extent, tested at the LHC by the SHIP and FASER experiments by studies of the $\nu_{\tau}$ neutrino production.