CT14 Intrinsic Charm Parton Distribution Functions from CTEQ-TEA Global Analysis

TL;DR

This work assesses whether a sizable intrinsic charm component can be inferred from a modern NNLO QCD global analysis (CT14/CT14HERA2) by testing valence-like BHPS and sea-like SEA models as nonperturbative inputs to the charm PDF. Using the S-ACOT-χ factorization scheme and a range of charm masses, the study finds that large intrinsic charm is disfavored, with 90% CL limits on the intrinsic charm momentum fraction at the initial scale around the sub-percentage level. EMC DIS data do not decisively constrain IC, and the impact of intrinsic charm on LHC observables such as Z and Higgs production is mild and model-dependent, often diluted by higher-order and multijet effects. The authors conclude that while intrinsic charm remains an intriguing possibility, its confirmation requires high-luminosity measurements and future facilities like an Electron-Ion Collider for definitive tests of NNLO-level charm scattering contributions.

Abstract

We investigate the possibility of a (sizable) nonperturbative contribution to the charm parton distribution function (PDF) in a nucleon, theoretical issues arising in its interpretation, and its potential impact on LHC scattering processes. The "fitted charm" PDF obtained in various QCD analyses contains a process-dependent component that is partly traced to power-suppressed radiative contributions in DIS and is generally different at the LHC. We discuss separation of the universal component of the nonperturbative charm from the rest of the radiative contributions and estimate its magnitude in the CT14 global QCD analysis at the next-to-next-to leading order in the QCD coupling strength, including the latest experimental data from HERA and the Large Hadron Collider. Models for the nonperturbative charm PDF are examined as a function of the charm quark mass and other parameters. The prospects for testing these models in the associated production of a Z boson and a charm jet at the LHC are studied under realistic assumptions, including effects of the final-state parton showering.

Figures (19)

• Figure 1: Leading-power (perturbative) radiative contributions for neutral-current DIS charm production and scattering, included up to ${\cal O}(\alpha_{s}^{2})$ in the S-ACOT-$\chi$ scheme. The figure is reproduced from Ref. Guzzi:2011ew.
• Figure 2: Dominant squared leading-power amplitudes in DIS charm production in the $Q^{2}\gg m_{c}^{2}\gg\Lambda^{2}$ limit. Here $F$ is the DIS structure function, $C_{0}$, $K_{0}$ and $D$ are two-particle irreducible (2PI) subgraphs, $T_{0}$ and $T_{0}^{(4)}$ are the twist-$2$ and $4$ target hadron subgraphs, and $K_{0}^{(2|4)}$ is the heavy-quark "mixed-twist" 2PI subgraph.
• Figure 3: Examples of subleading-power contributions to charm production originating from double-gluon initial states.
• Figure 4: $x c(x,Q)$ distributions for various models, evaluated at $Q=1.3$ GeV and $Q=2$ GeV, respectively.
• Figure 5: The change $\Delta\chi^{2}$ in the goodness of fit to the CT14 (left) and CT14HERA2 (right) data sets as a function of the charm momentum fraction $\langle{x}\rangle_{\rm IC}$ for the BHPS (blue) and SEA (red) models. Solid (dashed) lines represent the total $\chi^2$ and the partial $\chi^2_{global}$, as defined in Eq. (\ref{['chi2chi2P']}).