Semiclassical Virasoro Symmetry of the Quantum Gravity S-Matrix
  It is shown that the tree-level S-matrix for quantum gravity in four-dimensional Minkowski space has a Virasoro symmetry which acts on the conformal sphere at null infinity. 
Daniel Kapec, Vyacheslav Lysov, Sabrina Pasterski and Andrew   Strominger
hep-th gr-qc