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Glueballs

The mass region below 1500 MeV will probably be unraveled once the data from LEAR have been fully analysed. However, the definitive identification of f0(1500) as a glueball depends on the other $q\overline{q}$ scalars in particular the $s\overline{s}$ scalar which is still missing. All these states lie in the poorly known mass spectrum above 1600 MeV. Unfortunately, this region will not be accessible any longer at LEAR due to its recent closure. Glueballs may be distinguished from $q\overline{q}$ mesons in central production. The WA102 collaboration at CERN has discovered that $q\overline{q}$ meson production appears to be suppressed in certain kinematic domains and that the glueball signals survive. The f0(1500) is seen clearly supporting the glueball interpretation; an extensive investigation is now required. The first glueball excitation (2++) is expected to lie between 2.0 and 2.4 GeV. An intriguing, relatively narrow ($\sim $ 40 MeV), state, fJ(2220) (also known as $\xi $), has been observed by Mark III in radiative $J/\psi $ decay and is now also seen at Beijing in several two-pseudoscalar decay modes, with relative rates compatible with flavour blindness. Poor statistics does not allow a determination of its quantum numbers (spin 2 or 4) nor an accurate determination of its decay branching ratios. If J=2, this could be the first excited state of glue predicted by lattice gauge theories. All these issues could be addressed with high statistics data (i) in central collisions, i.e. with the COMPASS experiment at CERN from 1999, and (ii) in radiative $J/\psi $ decay at a $\tau$-charm factory. The other inspiration is that, if the f0(1500) has established the mass scale for excitation of the non-perturbative gluonic fields, it suggests that there should be new varieties of mesons, around 1.5 to 2 GeV, where the gluonic fields are excited in the presence of the quarks. These so called hybrid mesons may also now be emerging in experiments. One example is a state with exotic quantum numbers, JPC=1-+ that are unattainable from the excitation of quark degrees of freedom alone. Recently the experiments E852 at Brookhaven and Crystal Barrel at LEAR have found for a 1-+ state near 1400 MeV/c2 decaying into a $\eta\pi$-p wave which would be the first unambiguous proof for the existence of a state with exotic quantum numbers. Around 1900 MeV a new 2-+ state found by Crystal Barrel, $\eta_2(1875)$, has decay modes compatible with it being a hybrid state. Also in this interesting mass region, at 1.8 GeV, is an excitation of the pion, $ \pi(1800) $, whose decay properties are as predicted for hybrid excitations; there is also data from Serpukhov indicating that this state has significant decay into the glueball candidate ( $\pi(1800)\rightarrow \pi + f_0(1500)$). The possibility that the excited gluonic modes are being ``radiated'' as a glueball could have interesting implications for the electroproduction programme at the Jefferson Lab. Whereas mesons made from quarks may be produced at both the baryon and current (small t) vertices, a glueball can be produced dominantly at the baryonic vertex since it has no intrinsic coupling to photons. The study of the hybrid candidates will be made at the Jefferson Lab and also in a major programme in hadron spectroscopy with the COMPASS detector. 
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