The Charm Parton Content of the Nucleon

TL;DR

This work relaxes the common assumption that charm in the nucleon arises solely from perturbative gluon splitting, by introducing intrinsic charm (IC) as an independent nonperturbative component at μ0 ≈ m_c and testing three physically motivated shapes (BHPS, meson-cloud, sea-like) within a global QCD analysis. Using an updated framework with heavy-quark mass effects and full HERA I data, the authors constrain IC to lie between zero and roughly 0.02–0.024 in the charm momentum fraction, with model-dependent differences in the x-dependence. The results show that IC can substantially enhance charm distributions at moderate to large x, potentially affecting charm-initiated processes at the Tevatron and LHC, and motivates dedicated future measurements and the provision of IC-inclusive PDFs (CTEQ6.5C). Overall, the paper provides a systematic, data-driven bound on IC and clarifies how its presence would reshape charm-related phenomenology across energy scales. It also outlines experimental avenues, including collider studies and future ep colliders, to further pin down intrinsic charm.

Abstract

We investigate the charm sector of the nucleon structure phenomenologically, using the most up-to-date global QCD analysis. Going beyond the common assumption of purely radiatively generated charm, we explore possible degrees of freedom in the parton parameter space associated with nonperturbative (intrinsic) charm in the nucleon. Specifically, we explore the limits that can be placed on the intrinsic charm (IC) component, using all relevant hard-scattering data, according to scenarios in which the IC has a form predicted by light-cone wave function models; or a form similar to the light sea-quark distributions. We find that the range of IC is constrained to be from zero (no IC) to a level 2--3 times larger than previous model estimates. The behaviors of typical charm distributions within this range are described, and their implications for hadron collider phenomenology are briefly discussed.

Figures (7)

• Figure 1: Goodness-of-fit vs. momentum fraction of IC at the starting scale $\mu=1.3 \, \mathrm{GeV}$ for three models of IC: BHPS (solid curve); meson cloud (dashed curve); and sea-like (dotted curve). Round dots indicate the specific fits that are shown in Figs. \ref{['fig:figB']}--\ref{['fig:figD']}.
• Figure 2: Charm quark distributions from the BHPS IC model. The three panels correspond to scales $\mu = 2$, $\mu = 5$, and $\mu = 100 \, \mathrm{GeV}$. The long-dash (short-dash) curve corresponds to $\langle x \rangle_{c + \bar{c}} = 0.57\%$ ($2.0\%$). The solid curve and shaded region show the central value and uncertainty from CTEQ6.5, which contains no IC.
• Figure 3: Same as Fig. \ref{['fig:figB']}, except for the meson cloud model. The long-dash (short-dash) curves correspond to $\langle x \rangle_{c + \bar{c}} = 0.96\%$ ($1.9\%$).
• Figure 4: Same as Fig. \ref{['fig:figB']}, except for the sea-like scenario. The long-dash (short-dash) curves correspond to $\langle x \rangle_{c + \bar{c}} = 2.4\%$ ($1.1\%$).
• Figure 5: Comparison of charm with other flavors: $u, \bar{u}$ (long-dash, long-dash-dot); $d, \bar{d}$ (short-dash, short-dash-dot); $s = \bar{s}$ (dash-dot-dot), $g$ (dot). The solid curves are $c = \bar{c}$ with no IC (lowest) or the two magnitudes of IC in the BHPS model that are discussed in the text.