A QCD space-time analysis of quarkonium formation and evolution in hadronic collisions

The production of heavy quarkonium as QQbar bound-states in hadron-hadron collisions is considered within the framework of a space-time description, combining parton-cascade evolution with a coalescence model for bound-state formation. The `hard' production of the initial QQbar, directly or via gluon fragmentation and including both color-singlet and color-octet contributions, is calculated from the PQCD cross-sections. The subsequent development of the QQbar system is described within a space-time generalization of the DGLAP parton-evolution formalism in position- and momentum-space. The actual formation of the bound-states is accomplished through overlap of the QQbar pair and a spectrum of quarkonium wave-functions. This coalescence can only occur after sufficent gluon radiation reduces the QQbar relative velocity to a value commensurate with the non-relativistic kinematics of these bound systems. The presence of gluon participants in the cascade then is both necessary and leads to the natural inclusion of both color-singlet and color-octet mechanisms. The application of this approach to pp (ppbar) collisions from sqrt(s)= 30 GeV - 14 TeV reveals very decent agreement with available data from ISR and Tevatron - without the necessity of introducing fit parameters. Moreover, production probabilities are calculated for a complete spectrum of charmonium and bottonium states, with the relative significance compared to open charm (bottom) production. An analysis of the space-time development is carried through which sheds light on the relevance of gluon radiation and color-structure, suggesting a correponding experimental investigation.Comment: 37 pages including 16 postscript figure

Hadronization in Z0 decay

The confinement transition from the quark and gluon degrees of freedom appropriate in perturbation theory to the hadrons observed by real world experiments is poorly understood. In this strongly interacting transition regime we presently rely on models, which to varying degrees reflect possible scenarios for the QCD dynamics. Because of the absence of beam and target remnants, and the clean experimental conditions and high event rates, e+e- annihilation to hadrons at the Z0 provides a unique laboratory, both experimentally and theoretically, for the study of parton hadronization. This review discusses current theoretical understanding of the hadronization of partons, with particular emphasis on models of the non-perturbative phase, as implemented in Monte Carlo simulation programs. Experimental results at LEP and SLC are summarised and considered in the light of the models. Suggestions are given for further measurements which could help to produce more progress in understanding hadronization.Comment: Topical review, to appear in J.Phys.G, 80 page

Evolution Effects in ... Fragmentation into Charmonium

In Z 0 decay into prompt charmonium, i.e. charmonium not originating from B-meson decays, the most important contribution is expected to come from colour-octet mechanisms. However, previous fixed-order calculations of the colour-octet contribution contain large logarithms which, in a more complete treatment, should be resummed to all orders. We study this resummation by using a Monte Carlo QCD cascade model and find that the fixed-order colour-octet result is diminished by 15%. We compare the Monte Carlo calculations with results obtained by using analytical evolution equations. 1 Introduction The production of charmonium and bottomonium states in various processes, especially at high-energy colliders [1], has recently received considerable experimental and theoretical interest. New data have become available from p¯p [2], ep [3] and e + e \Gamma [4--6] colliders. Theoretically, it has been realized that quarkonium production at colliders is dominated by parton fragmentation ..