Central Production of Exotics

Abstract

The availability of glueball studies with the COMPASS setup is presented. Central production of X decaying into ηη was used to estimate the registration efficiency. A few scenarios of the possible setup geometry are compared. 1. PHYSICS MOTIVATION Spectroscopy of light quark systems and glueballs is one of the goals of the COMPASS programme. Since the experiment proposal was published six years ago new experimental data and theoretical descriptions have appeared. But still there are a lot of open questions in this field, high-statistics detailed data are needed to clarify the picture. A detailed review of this subject was made by S. Godfrey at this workshop, so my presentation will be short. QCD predicts the existence of nonq q mesons such as glueballs and hybrids. The best glueball mass estimates come from lattice gauge theory calculations. The lightest glueball has J PC = 0 and its mass should be in the range 1.45–1.75 GeV. Special methods based on production characteristics, decay patterns and relations to other mesons could be applied for a glueball search, namely: • search for the states with J PC not allowed for normal q q states, for example 1; • a study of the extra states, that is states that have the quantum numbers of already completed nonets, with low masses (to exclude radially excited nonet members); • a detailed study and look for the states with unusual branching ratios; • search for the states preferentially produced in gluon-rich processes (Fig. 1): Pomeron– Pomeron scattering, J/ψ decays, proton–antiproton annihilation, special hadronic reactions. Fig. 1: Gluon-rich processes: a) Pomeron–Pomeron exchange; b) and c) J/ψ decays; d) p p annihilation; e) reactions involving disconnected quark lines. According to lattice inspired models, glueballs will mix strongly with nearby q q states with the same J . The three states in the glueball mass range are: f0 (1370), f0 (1500) and f0 (1710). The WA102 Collaboration published, for the first time in a single experiment, a complete data set for the decay branching ratios of these mesons to all pseudoscalar meson pairs: , η η ηη, , K K ππ, ′ 4π. Based on this data, an analysis of the scalar glueballq q mixing was done by A. Kirk and F. E. Close [1]. They identify a systematic correlation between glueball mass, mixing, and flavour symmetry breaking and conclude that the glueball may be rather lighter than some quenched lattice QCD computations have suggested. A result that is more general than any specific mixing scheme is that no pair out of the three f0 (1370), f0 (1500), f0 (1710) can be in the same pure q q nonet; other degrees of freedom are required. The WA102 data and world averages lead to the summary for the favoured results for which mG = 1443 ± 24 MeV, mN = 1377 ± 20 MeV and mS = 1674 ± 10 MeV. The solution is compatible with the relative production strength in pp central production, p p -annihilations and J/ψ radiative decays. An interesting empirical observation of a different topology for central production of glueball candidates and q q mesons was done [2]. It was observed, that the ratio GeV) 5 . 0 ( GeV) 2 . 0 (