This picture of the scalar mesons seems to fit experimental results well in certain ways, but often receives harsh criticism for ignoring unsolved problems with chiral symmetry breaking and the possibility of a non-trivial vacuum state as suggested by Gribov.
[5] Many attempts have been made to determine the quark content of the lighter scalar mesons; however, no consensus has yet been reached.
The scalar mesons in the mass range of 1 GeV/c2 to 2 GeV/c2 are generally believed to be conventional quark-antiquark states with orbital excitation L = 1 and spin excitation S = 1,[6] although they occur at a higher mass than one would expect in the framework of mass-splittings from spin–orbit coupling.
[7] The scalar glueball[8] is also expected to fall in this mass region, appearing in similar fashion to the conventional mesons but having very distinctive decay characteristics.
The scalar mesons in the mass range below 1 GeV/c2 are much more controversial, and may be interpreted in a number of different ways.