Abstract
We discuss the role of intrinsic charm (IC) in the nucleon for forward production of c-quark (or overline{c} -antiquark) in proton-proton collisions for low and high energies. The calculations are performed in collinear-factorization approach with on-shell partons, kT-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 coverline{c} -pair production at large rapidities or Feynman-xF. We perform similar calculations within leading-order and next-to-leading order kT-factorization approach. The kT-factorization approach leads to much larger cross sections than the LO collinear approach. At high energies and large rapidities of c-quark or overline{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 ντ neutrino production.
Highlights
We discuss the role of intrinsic charm (IC) in the nucleon for forward production of c-quark in proton-proton collisions for low and high energies
We see that the predictions for charm quark transverse momentum and rapidity distributions are very sensitive to the choice of this parameter, especially, at small charm quark transverse momenta, which affects the rapidity spectrum
For a better transparency in figure 17 we compare the hybrid model results obtained with the KMR-CT14lo with the PB-next-to-leading order (NLO)-set1 unintegrated parton distribution functions (uPDFs), that correspond to the two different hybrid calculation schemes, together with the results obtained in the collinear approach
Summary
In the five-quark Fock component uudccheavy quark/antiquark carries rather large fraction of the mother proton. Such an approach leads to c(x) = c(x) In practice both models give rather similar distributions as will be shown in the following, so using one of them as an example is representative and sufficient. The IC may have small-x component known under the name of sea-like, only simple ad hoc parametrizations were used in the literature There is another category of processes leading to sea-like IC (see figure 1 where an example of corresponding dynamical processes is shown). In the GRV approach [29] the charm contribution is calculated fully radiatively as the convolution of gluon distribution with appropriate mass-dependent splitting function: xc(x, Q2) = αs(μ 2). The explicit formula for Cgc, and a, including mass of quarks/antiquarks, can be found in ref. In the right panel figure 4 we show charm distribution in a proton without (dashed line) and with (solid line) the IC distribution taken as initial condition of the evolution
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