ar X iv : h ep - p h / 98 03 32 5 v 1 1 2 M ar 1 99 8 Leading Particle Effect in the J / ψ Elasticity Distribution

Recent DESY-HERA data on J/Ψ elasticity distribution show that it emerges mostly as a fast particle. Interpreting photoproduction as a collision between a pre-formed charmed hadron and the proton, the out-coming J/Ψ is a leading particle of the collision. We analyse these data using a model formulated to describe energy flow in hadron-hadron reactions. The measured J/Ψ spectrum can be successfully described in terms of this model. We conclude that the observed transparency of the charmed hadron-proton collisions arises because of the particularly small gluonic content of the initial c − c state. The leading particle effect is one of the most interesting features of multiparticle production in hadron-hadron collisions [1]. In these reactions the valence quarks of the projectile emerge from the collision carrying the initial state quantum numbers. All produced particles come essentially from the gluons and quark-anti-quark pairs already pre-existing in the projectile and target, or radiated during the collision. This qualitative picture takes different implementations in the many existing multiparticle production models. In one of them, the Interacting Gluon Model (IGM) [2, 3, 4, 5], the produced particles (and consequently the energy released in the secondaries and lost by the projectiles) come almost entirely from the pre-existing gluons in the incoming hadrons. This conjecture may be directly tested using a high energy, nearly gluonless hadronic projectile. In this case, according to the IGM, inspite of the high energy involved, the production of secondaries would be suppressed (in comparison to the production observed in ordinary hadron induced reactions) and the energy would be mostly carried away by the projectile leading particle (LP) which would then be observed with a hard x F spectrum. This type of gluonless projectile is available in J/Ψ photoproduction, where the photon can be understood as a virtual c − c pair which reacts with the proton and turns into the finally observed J/Ψ. There are low energy data taken by the FTPS Collaboration [6] and very recently high energy data became available at HERA [7]. It has been shown by a number of authors that the simple vector meson dominance mechanism (VDM), in which the photon is converted directly to a J/Ψ before the interaction with the proton, does not describe many aspects of data. However, as it was shown in ref. [8], the charm anti-charm pair can be understood as a supperposition of many hadronic states: J/Ψ, Ψ ′ , .... …