Massless Gauge Bosons other than the Photon
  Gauge bosons associated with unbroken gauge symmetries, under which all standard model fields are singlets, may interact with ordinary matter via higher-dimensional operators. A complete set of dimension-six operators involving a massless U(1) field, gamma-prime, and standard model fields is presented. The mu to e gamma-prime decay, primordial nucleosynthesis, star cooling and other phenomena set lower limits on the scale of chirality-flip operators in the 1 - 15 TeV range, if the operators have coefficients given by the corresponding Yukawa couplings. Simple renormalizable models induce gamma-prime interactions with leptons or quarks at two loops, and may provide a cold dark matter candidate. 
Bogdan A. Dobrescu