Difficulties with Photonic Searches for Magnetic Monopoles
  Recently, there have been proposals that the classic Euler-Heisenberg Lagrangian together with duality could be employed to set limits on magnetic monopoles having masses less than 1 TeV. The D0 collaboration at Fermilab has used such a proposal to set mass limits based on the nonobservation of pairs of photons each with high transverse momentum. In this note, we critique the underlying theory, by showing that at the quoted limits the cross section violates unitarity and is unstable with respect to radiative corrections. It is proposed that the correct coupling of magnetic monopoles to photons leads to an effective softening of the interaction, leading to a much smaller cross section, from which no significant limit can be obtained from the current experiments. Previous limits based on virtual monopole loops are similarly criticized. 
Leonard Gamberg, George R. Kalbfleisch, and Kimball A. Milton