Gluon induced contributions to Z gamma production at hadron colliders
  We study the contribution of gluon induced partonic subprocesses to Z gamma pair production at hadron colliders. These processes contribute only at next-to-next-to-leading order but are potentially enhanced by two factors of the gluon parton densities. However, we find that their contribution is modest and that next-to-leading order calculations give reliable predictions. 
K. L. Adamson, D. de Florian, A. Signer