Searching for the Scalar Glueball

Abstract

Existence of gluonic resonances is among the early expectations of QCD. Today, QCD calculations predict the lightest glueball to be a scalar state with mass within a range of about 900-1700 MeV but there is no consensus about its experimental evidence. In a re-analysis of the phase shifts for pi pi scattering up to 1800 MeV where such states should show up we find the broad resonance f_0(600)/\sigma contributing to the full mass range and the narrow f_0(980) and f_0(1500) but no evidence for f_0(1370). Phenomenological arguments for the broad state to be a glueball are recalled. It is argued that the large radiative width of f_0(600)/\sigma reported recently is not in contradiction to this hypothesis but is mainly due to pi pi rescattering. The small ``direct'' radiative component is consistent with QCD sum rule predictions for the light glueball.