Confinement, Crossing Symmetry, and Glueballs
  We suggest that the quark-confining force is related by crossing symmetry to a color-singlet glueball ${\cal G}$ which is well described as a loop of one quantum of color magnetic flux. Electron pair annihilation as high as $\approx 2 GeV$ above the $\Upsilon$ mass could produce $\Upsilon \rightarrow \ell^+\ell^-$ accompanied by ${\cal G}$ or one of its excited states. 
Alfred S. Goldhaber and T. Goldman