QCD and spectroscopy beyond the quark model : the search for the lightest scalar glueball
The naive quark model has been remarkably successful in classifying the many hadron states so far discovered. The underlying theory for these successes is QCD. This theory, however, not only suggests states made purely of quarks, but also those containing glue. Whether these pure glue states are observable depends on the validity of the notion of valence gluons which has been explored in the earlier part of this thesis. My studies show that the hard and soft gluon components can be distinguished and behave as valence and sea constituents in glueballs. These glueballs, and especially the ground state, must be found in order to establish QCD as a successful theory of strong interactions. Because of the inevitability of mixing, particularly for low mass states, simple parton configurations cannot be expected in practice, so supposedly characteristic decay patterns may not serve as a guide to the existence of new states. The only reliable way to establish the intrusion of extra dynamics is to count the number of states with given quantum numbers. An extensive coupled channel analysis of results on ππ and KK final states interactions with I≈0 0(^++) quantum numbers below 1.7 GeV has been performed incorporating new data on pp ÷ pp ππ(KK), Though no poles are imposed on these data, we find that 3 distinct resonances emerge in the 1 GeV region, when the naive quark model requires but two. This clearly indicates for the first time definite evidence for dynamics beyond the quark model in the 0(^++) channel. Our results are consistent with the presence of the ground state glueball S(_1)(993) together with ideally mixed quark model states S(_2)(988) and ε(900).