Positivity of Curvature-Squared Corrections in Gravity
  We study the Gauss-Bonnet (GB) term as the leading higher-curvature correction to pure Einstein gravity. Assuming a tree-level ultraviolet completion free of ghosts or tachyons, we prove that the GB term has a nonnegative coefficient in dimensions greater than four. Our result follows from unitarity of the spectral representation for a general ultraviolet completion of the GB term. 
Clifford Cheung, Grant N. Remmen
hep-th gr-qc hep-ph