Extended Double Lattice BRST, Curci-Ferrari Mass and the Neuberger   Problem
  We present Extended Double BRST on the lattice and extend the Neuberger problem to include the ghost/anti-ghost symmetric formulation of the non-linear covariant Curci-Ferrari (CF) gauges. We then show how a CF mass regulates the 0/0 indeterminate form of physical observables, as observed by Neuberger, and discuss the gauge-parameter and mass dependence of the model. 
M. Ghiotti, L. von Smekal and A.G. Williams