The presence of an η (1405) state also brings glueball mixing into the discussion. It is possible that the η and η ′ mesons mix with the pseudoscalar glueball which should occur somewhere above the scalar glueball in mass, as an unmixed state. This is one of a few ways in which the unexpectedly large η ′ mass of 957.78 MeV/c can be explained, relative to its model-predicted mass around 250–300 MeV/c.