Leading particle effect in the J/<FONT FACE=Symbol>y</font> elasticity distribution

Abstract

Recent DESY-HERA data on J= 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 outcoming J= is a leading particle of the collision. We analyse these data using a model formulated to describe energy ow in hadron-hadron reactions. The measured J= 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.PACS numbers: 13.85.Qk, 11.55.JyThe leading particle e ect is one of the most interesting features of multiparticle production in hadronhadron 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 quarkanti-quark pairs already pre-existing in the projectile and target, or radiated during the collision. This qualitative picture takes di erent 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= photoproduction, where the photon can be understood as a virtual c , c pair which reacts with the proton and turns into the nally observed J= . 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 n umber of authors that the simple vector meson dominance mechanism VDM, in which the photon is converted directly to a J= 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= , 0 , ... . Whereas relevant for the calculation of the cross sections, the inclusion of all the other charmonium states does not change the fact that the projectile, whatever it may be, is a hadron with low gluonic content. For the study of the outgoing J= momentum spectrum, this is the most important information, which allows us to

The Interacting Gluon Model: a review

“…16 we show the ratio R(M X ) defined by: (19) This quantity involves only distributions previously normalized to unity and does not directly compare the cross sections (which are numerically very different for DPE and single diffraction). In R the dominant 1/M 2 X factors cancel and we can better analyse the details of the distributions which may contain interesting dynamical information.…”

“…Its original application to the description of inelasticity [12] and multiparticle production processes in hydrodynamical treatments [13] was followed by more refined applications to leading charm production [14] and to single diffraction dissociation, both in hadronic reactions [15] and in reactions originated by photons [16]. These works allowed for providing the systematic description of the leading particle spectra [17] and clearly demonstrated that they are very sensitive to the amount of gluonic component in the diffracted hadron as observed in [18] and [19]. We have found it remarkable that all the results above were obtained using the same set of basic parameters with differences arising essentially only because of the different kinematical limits present in each particular application.…”