Evidence for the Observation of a Glueball

The existence of chromoelectric field is the key hypothesis QCD adds to the quark model. The observation in experiment of a glueball would be a direct confirmation of the existence of chromoelectric field. So it would be nice to find one. Identifying a glueball in experiment is tough, however. The kinematics of QCD provides no clear footprint for glueballs comparable, say, to the footprint the Weinberg-Salam model gives for the W’s and the Z. The lightest states, which would be easiest to detect, are not expected to differ drastically in mass or decay properties from mesons containing quarks and antiquarks. On the other hand, J combinations which are impossible for quark-antiquark states but which can occur for heavier glueball excitations can also be realized with multiple quark-antiquark pairs. A reliable calculation of the consequences of QCD’s dynamics for the masses and decay properties of glueballs appears to be a necessary element in the identification of glueballs in experiment. At present such calculations can be done only numerically, using the lattice formulation of QCD in combination with the valence approximation. As of three years ago, two independent calculations had been completed of the infinite-volume, continuum limit of the mass of the lightest glueball, which turns out to be a scalar. Hong Chen, Jim Sexton, Alessandro Vaccarino and I obtained 1 a value of 1740(71) MeV using ensembles of 25000 to 30000 gauge configurations on each of several different lattices. An earlier valence approximation calculation by the UKQCD-Wuppertal collaboration , when extrapolated to the continuum limit, yields 1625(94) MeV for the lightest scalar glueball mass. This calculation used several different lattices with ensembles of between 1000 and 3000 configurations each. If the two mass evaluations are combined, taking into account the correlations between their statistical uncertainties arising from a common procedure for converting lattice quantities into physical units, the result is 1707(64) MeV for the scalar glueball