Is the $\sigma(600)$ a Glueball? Two photon reactions can tell us
  Minkowski and Ochs have recently argued that the small two photon coupling of a conjectured $\sigma(600)$ is so small that it is likely to be a glueball. We ask whether this can be so or whether it is simply gauge invariance that produces the observed low mass suppression? 
M.R. Pennington