On the Intrinsic Charm Component of the Nucleon

Using a D meson cloud model we calculate the squared charm radius of the nucleon . The ratio between this squared radius and the ordinary baryon squared radius is identified with the probability of “seeing” the intrinsic charm component of the nucleon. Our estimate is compatible with those used to successfully describe the charm production phenomenology. In the early eighties there was a hope to understand charm production solely in terms of perturbative QCD. Inspite of all uncertainties in defining the scale , it would be in any case of the order of a few GeV and therefore the coupling constant would be smaller than one. As more and more data became available it became clear that perturbative QCD alone was not enough to properly account for the measured differential cross sections. Higher order corrections did not make the situation any better. The main problem was that the produced charmed particles were too fast. In other words, there was a remarkable excess of particles with large Feynman x ( xF ). Already at that time, the idea was advanced [1], that the hadron wave function contains a charm component even before undergoing a collision. This component is originated in higher twist QCD interactions inside the hadron. The so called “intrinsic” charmed pairs produced by these interactions have nothing to do with usual sea quark pairs. The crucial difference between them is that the intrinsic charm is part of the valence system and therefore very fast in contrast to the sea charm, which is slow. During the last years, an intrinsic charm component was added to the perturbative QCD component in a quantitative and systematic way [2] . As a result , a very good description of data was achieved. In order to obtain such good agreement with experimental data the crucial point was the normalization of the intrinsic charm component , σic of the hadron + nucleon → c − c X cross section . The quantity σic is related to the probability of observing the intrinsic charm component of the hadron, Pic [3]. It is very difficult (if not impossible) to calculate this quantity from first principles. It was estimated from a phenomenological analysis to be less than 1% [4]. In fact, Pic = 0.3% seems to be the best value to describe recent data on charm production [2]. A very important question is, of course, whether this 1% of intrinsic charm can be supported by any model based calculation. In ref. [5], such a calculation was done using the MIT bag model. It was found that the probability of finding a five-quark component