Experimental signatures of a massive mirror photon
  There has been some speculation about the possible existence of a second photon which only couples to ordinary matter through mixing of the gauge kinetic terms. If the exotic photon is massless, then the principle phenomenological effect of the kinetic mixing is to give the mirror particles small electric charges (mini-charged particles). We examine the possibility that the exotic photon is massive. In this case the experimental signatures of the mirror photon are quite different to the case when the exotic photon is massless. In this scenario you do not get mini charged fermions but instead you can get processes directly producing the mirror photon such as Compton-like scattering: $e+\gamma \rightarrow e + \gamma^m$ (where $e, \gamma, \gamma^m$ denote the electron, photon and mirror photon respectively). 
R. Foot