Entanglement Entropy for Relevant and Geometric Perturbations
  We continue the study of entanglement entropy for a QFT through a perturbative expansion of the path integral definition of the reduced density matrix. The universal entanglement entropy for a CFT perturbed by a relevant operator is calculated to second order in the coupling. We also explore the geometric dependence of entanglement entropy for a deformed planar entangling surface, finding surprises at second order. 
Vladimir Rosenhaus and Michael Smolkin
hep-th cond-mat.stat-mech gr-qc quant-ph