The case of the pseudoscalar glueball

Glueballs represent a key requirement of quantum chromodynamics as a non-Abelian field theory. Their search provides one of the strongest motivations for meson spectroscopy. The first glueball candidate was identified in 1980 in the J/Psi radiative decay. Its discovery actually dates back to 1963 and for four decades about 30 experiments, using six different production mechanisms, were dedicated to studying the pseudoscalar states lying in the 1.4-1.5 GeV/c(2) mass region. Today, the presence of two pseudoscalar states and an axial vector can be considered as established (see 2004 edition of the Review of Particle Properties). Assuming that.(1295) is established and the nonet filled, the lower mass pseudoscalar,.(1405), becomes a supernumerary and shows the properties of a non-(q) over barq state. Here, we review the experimental effort related to this long search, which can be considered a sort of paradigm for light-quark spectroscopy.