HADRON 95: Looking hard for glueballs

Gluons, the particles which mediate inter-quark forces, should in principle form particles - 'glueballs' - which complement the familiar subnuclear particles built from quarks. Recent progress in the search for glueballs was a major topic at the 6th International Conference on Hadron Spectroscopy (Hadron 95) held at the University of Manchester. While bound states of gluons are firmly predicted by quark-gluon field theory (quantum chromodynamics - QCD), until recently QCD lattice calculations have been unable to provide reliable predictions of glueball masses and widths, while only tentative and controversial experimental evidence existed. However studies of gluon-rich processes such as low energy proton-antiproton annihilation, central production in diffractive collisions and radiative J/psi decays have revealed states difficult to accommodate in the conventional quark-antiquark meson classification. Also, the calculations by different lattice QCD groups are now beginning to give consistent results. The recent experimental and theoretical progress presented at the conference prompted Frank Close of the Rutherford Appleton Laboratory to remark ''it may be the case that we are at last beginning to see things''. Evidence is now accumulating that the spin-parity 0⁺⁺ glueball has been observed, as the f₀(1500), in a number of different gluon-rich experimental situations. Close argued against the widely-held view that meson flavour symmetry - the approximately equal production of strange and non-strange particles - should be expected in glueball decays. If, as seems likely, the 0⁺⁺ states are close in mass to the conventional 0⁺⁺ mesons, then mixing between the glueball and the quark states would remove the flavour symmetry. He went on to argue that the f₀, with a width of only about 100 MeV, is too narrow to be a quark-antiquark bound state and that its decays have a ''gluish'' nature. However the 2⁺⁺ glueballs are expected to be well separated in mass from the corresponding meson nonet and should look very different to conventional mesons. New results on radiative J/psi decays were presented by Shan Jin on behalf of the BES collaboration at the Beijing electron-positron Collider. In 1984, the Mark III Collaboration at Stanford's SPEAR electron-positron collider reported a narrow state, the xi(2230), decaying into a pair of neutral kaons, although no other decay modes were seen and no other experiment had since been able to confirm it