A covariant constituent quark/gluon model for the glueball-quarkonia   content of scalar-isoscalar mesons
  We analyze the mixing of the scalar glueball with the scalar-isoscalar quarkonia states above 1 GeV in a non-local covariant constituent approach. Similarities and differences to the point-particle Klein-Gordon limit and to the quantum mechanical case are elaborated. Model predictions for the two-photon decay rates in the covariant mixing scheme are indicated. 
F. Giacosa, T. Gutsche and Amand Faessler