THE CRYSTAL BARREL EXPERIMENT AT LEAR
The Crystal Barrel Experiment ran at the Low Energy Antiproton Ring, (LEAR), from 1989 through the end of 1996 and studied antiproton proton and antiproton deuterium annihilations both at rest and in flight. The detector covered a solid angle of nearly 4π, and provided high resolution systems for both charged particles and photons.
One of the main goals of the experiment was to search for gluonic excitations in the meson spectrum. This includes both glueballs, and hybrid mesons. Of particular interest is the scalar glueball candidate, the f0(1500), which Crystal Barrel discovered. This state has been studied in detail, and its peculiar decay pattern mapped out. In particular, this decay pattern to pairs of pseudoscalar mesons makes this a prome candidate for the groundstate scalar glueball, particularly if it is mixed with the normal scalar mesons.
Crystal Barrel has also observed a particle with explicitly exotic quantum numbers, now named π1(1400), decaying into an eta and a pion. The state with mass near 1400 MeV/c2 and width of about 300 MeV/c2 does not fit most current models of a hybrid mesons, but its 1-+ quantum numbers mean that this state is clearly something beyond the quark-antiquark picture of mesons.