ON THE INTRINSIC CHARM COMPONENT OF THE NUCLEON.pdf

P o s t S c r i p t? p r o c e s s e d b y t h e S L A C / D E S Y L i b r a r i e s o n 2 5 A p r 1 9 9 5 . H E P - P H - 9 5 0 4 3 8 8 ON THE INTRINSIC CHARM COMPONENT OF THENUCLEONF. S. NAVARRA, M. NIELSEN, C. A. A. NUNES and M. TEIXEIRANuclear Theory and Elementary Particle Phenomenology GroupInstituto de Fsica da Universidade de S~ao Paulo,Caixa Postal 20516, 01498-970 S~ao Paulo, Brazil Abstract:Using a D meson cloud model we calculate the squared charm radius of thenucleon . The ratio between this squared radius and the ordinary baryon squared radiusis identied with the probability of \seeing the intrinsic charm component of the nucleon.Our estimate is compatible with those used to successfully describe the charm productionphenomenology. In the early eighties there was a hope to understand charm production solely in terms ofperturbative QCD. Inspite of all uncertainties in dening the scale , it would be in any caseof 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 wasnot enough to properly account for the measured dierential cross sections. Higher ordercorrections did not make the situation any better. The main problem was that the producedcharmed particles were too fast. In other words, there was a remarkable excess of particleswith large Feynman x ( xF ).Already at that time, the idea was advanced [1], that the hadron wave function contains acharm component even before undergoing a collision. This component is originated in highertwist QCD interactions inside the hadron. The so called \intrinsic charmed pairs producedby these interactions have nothing to do with usual sea quark pairs. The crucial dierencebetween them is that the intrinsic charm is part of the valence system and therefore veryfast in contrast to the sea charm, which is slow. During the last years, an intrinsic charmcomponent was added to the perturbative QCD