Comment on "New Limits on Intrinsic Charm in the Nucleon from Global Analysis of Parton Distributions".

Intrinsic heavy quarks in hadrons emerge from the non-perturbative structure of a hadron bound state [1] and are a rigorous prediction of QCD [2, 3]. Lattice QCD calculations indicate significant intrinsic charm and strangeness probabilities [4, 5]. Since the light-front momentum distribution of the Fock states is maximal at equal rapidity, intrinsic heavy quarks carry significant fractions of the hadron momentum. The presence of Fock states with intrinsic strange, charm, or bottom quarks in hadrons lead to an array of novel physics phenomena [6–8]. Accurate determinations of the heavy-quark distributions in the proton are needed to interpret Tevatron and LHC measurements as probes of physics beyond the Standard Model [9, 10]. Determinations [9, 11, 12] of the momentum fraction carried by intrinsic charm quarks in the proton typically limit x IC ∼ O(1%) at 90% CL, consistent with the EMC analysis of their charm structure function data [13] and the large rate for high-p T ¯ pp → cγX reactions at the Tevatron [14]; however, a precise determination of x IC has proven elusive. The recent letter by P. Jimenez-Delgado et al. (JDHLM) [15] reports x IC = (0.15 ± 0.09)%. The authors include low-energy data from the ed (p) → e ′ X SLAC experiment [16] in their global fit. They find that this data set places strong constraints on intrinsic charm, although, by their count, only 157 of 1021 data points have W 2 in excess of the charm hadronic threshold: W 2 th ≈ 16 GeV 2. The SLAC measurements of ed (p) → e ′ X have an overall normalization (systematic) error of ± 1.7 (2.1)%, and a relative normalization error of typically ±1.1% [16]. The SLAC data points in the germane regime of W 2 16 GeV 2 , x > 0.1 have even larger statistical uncertainties. It is clearly challenging to identify the contribution from charm quarks to the inclusive structure function if only the scattered electron is detected. In addition to the valence and sea-quark distributions, there are other contributions to the inclusive cross section which need to be determined to high accuracy in order to discern the intrinsic charm component at the level claimed by JDHLM; this includes higher-twist corrections at high x, the strange and bottom quark contributions, as well as the accurate implementation of the suppression of charm production at threshold and nuclear target effects. Their analysis depends on uncertain …