The case of the pseudoscalar glueball

The η(1405), once called the E or ι represents a case in itself in the glueball search. It has been studied through six different production mechanisms by more than thirty experiments for a period spanning four decades. More than 70 experimental results are now quoted within the Review of Particle Properties. The purpose of this review is to focus on the key point: the existence or not of an extra pseudoscalar state . A possible interpretation of the data indicating the presence of a non qq-bar state will be presented and discussed. Open questions and alternative interpretations will be considered. (author)