Charm in the Nucleon

TL;DR

This work evaluates whether intrinsic charm exists in the nucleon by performing a next-to-leading order analysis of charm electroproduction across $x$ and $Q^2$ using three schemes that interpolate between massless evolution and photon–gluon fusion. The authors implement a comprehensive framework—the VFNS, FFNS, and a mass-aware interpolating scheme—that includes quark and target mass effects, threshold behavior, and potentially non-perturbative charm, applying them to $F_2^{c}(x,Q^2)$. They find that perturbative charm suffices in much of the data, but several large-$x$ EMC points resist such explanations; introducing intrinsic charm helps in some cases but cannot consistently fit all measurements with a single IC scenario. The analysis underscores the necessity of properly accounting for heavy-quark masses and thresholds and indicates that, while intrinsic charm remains a possibility at the few-per-mille level, the current data do not provide conclusive evidence, motivating improved high-$x$ measurements and further theoretical refinement.

Abstract

A next-to-leading order analysis of inelastic electroproduction of charm is performed using an interpolating scheme which maps smoothly onto massless QCD evolution at large Q^2 and photon-gluon fusion at small Q^2. In contrast with earlier analyses, this scheme allows the inclusion of quark and target mass effects and heavy quark thresholds, as well as possible non-perturbative, or intrinsic, charm contributions. We find no conclusive evidence in favor of an intrinsic charm component in the nucleon, although several data points which disagree with perturbative QCD expectations will need to be checked by future experiments.

Figures (9)

• Figure 1: Photon--gluon fusion process at leading order in $\alpha_s$.
• Figure 2: Charm structure function as a function of $Q^2$ for $x=0.05$ and 0.2, evaluated according to the various schemes discussed in the text. "IS" refers to the interpolating scheme of Eq.(\ref{['interp']}), while "IS (massive $g$)" denotes the interpolating scheme but with massless quark coefficient functions. The GRV parameterization GRV for the gluon is used.
• Figure 3: Charm structure function calculated within the interpolating scheme for different gluon distributions, GRV GRV (dotted) and MRST MRST with minimum (dashed) and maximum (solid) gluons. The data at small $x$ are from the ZEUS Collaboration ZEUS, while the large-$x$ data are from the EMC EMC. For clarity the small-$x$ curves have been scaled by a factor 10 (0.1) for $Q^2=25$ (60) GeV$^2$, and the large-$x$ curves by a factor 100 (0.01).
• Figure 4: Charm quark distributions from the intrinsic charm models IC1 MT (solid) and IC2 BROD (dashed), both normalized to 1%, and from the MRST parameterization MRST (with maximal gluon) at $Q^2=5$ GeV$^2$ (dotted).
• Figure 5: The charm structure function calculated through the interpolating scheme ("IS") in Eq.(\ref{['interp']}) for intrinsic charm distributions from models IC1 and IC2, normalized to 1%. The GRV parameterizations GRV for the gluon and light quark densities are used.