Scalar mesons: in search of the lightest glueball

Abstract

According to the QCD expectations the lightest glueball should be a scalar particle (J^{PC}=0^{++}). Different scenarios have been considered for a classification of these states but -- despite considerable progress in recent years -- the experimental basis for various parameters is still rather weak. We present a new analysis of the elastic and charge exchange pi pi scattering between 1000 and 1800 MeV. A unique solution is selected which shows clear evidence for f_0(1500) and a broad state (\sigma or ``f_0(1000)''), but there is no evidence for f_0(1370) at a level of \gtrsim 10% branching ratio to pi pi. Arguments in favour of the broad state to be a glueball are summarized.