Glueballs, η and η′ Mesons and Antiproton-Proton Annihilation Dynamics

Abstract

A new scalar isoscalar state, the f ₀(1500), has been discovered in a series of recent experiments, in $\bar pp$ annihilation at rest and in flight, in charge-exchange, Pomeron-Pomeron scattering, and in radiative J/ψ decays. The state has large decay branching ratios to ππ, σσ, ηη, and to ηη′. The production characteristics and the decay modes suggest an interpretation of the f ₀(1500) as ground-state scalar glueball even though other interpretations cannot yet be excluded. The experimental situation is reviewed with particular emphasis on the results obtained by the Crystal Barrel Collaboration at LEAR in $\bar pp$ annihilation into 3 pseudoscalar mesons.

In $\bar pp$ annihilation at rest into two final-state mesons dynamical selection rules have been observed, the best-known example being the ρπ puzzle. Even more striking examples are found in strange-meson production. These dynamical selection rules require an interpretation in terms of symmetries of the underlying dynamical amplitudes. In it assumed that this symmetry is SU(3). However, SU(3) can be formulated in different coupling schemes; in terms of the underlying quark dynamics, of s-channel resonances or in terms of baryon exchanges in the t or u channel. The different (squared) amplitudes are compared with experimental data.